Just to answer a few of the questions (I was there last month(US citizen/GCC resident went to Salalah):
1. They do accept the sister countries visas (I know for sure Qatar with the stamp they do, but not 100% sure about Dubai)
2. You can get VOA at Salalah. Supposedly you can also do it online, although I tried like 50 times and it failed.
3. As indicated, the visa is 5OR for tourist visa short term for US Citizens or GCC residents, and 20 OR for longer term. You just pay at the visa desk and then head to the passports.

For info.

I applied for a online Tourist Visa. No problems with the process.
I'm from Oz and used Oz Mastercard for payment.

I ended up with a VOA email from ROP that is both a Visa Application and a Receipt. It can only be done a max of 3 months prior to your arrival in Oman and must be presented (the email PDF) at Immigration on arrival.

Am I right in thinking that the short term Oman visa is no longer available and everyone has to get the longer term visa for OMR20? If so, that's a major downer.

The Omani police have "upgraded" their website and I can't see any visa-on-arrival information in English any more, although some links do redirect to the "old" site.

OMR 20 = USD 56 which is quite expensive. Rather spend the money in the country itself...

But now that Qatar is visa-free for many western countries, can one avoid paying for an Omani visa by flying QR and passing through immigration at DOH? Will it be awkward to go in and out of Qatar on a short transit, and also not sure how to convince Oman immigration that this exists if they doubt it - any Arabic readers able to find a page which could be printed out that will at least encourage them to investigate further?

I will be on the LX flight from Zurich which makes a stop in Dubai. But I don't suppose that will count. Been to the UAE earlier this year, in March, but that's probably too far back.

You need to enter the UAE at Dubai, and then within the 30-day period granted for the UAE, fly directly to Oman

People (who have the right passport and are) joining the LX flight at DXB won't need an Omani visa but if you are coming from ZRH and not getting off you will.

I await anyone who can report whether entering Qatar first now works

They can be surprisingly forgiving in their interpretation of the "Dubai rule". I'd been in Dubai but arrived from Sharjah, and was told the free visa was only for someone coming directly from a DXB visit (which i think answers your question). Yet Mr Laid Back gave a smile, said welcome to Oman and stamped my passport.

The problem is you'd have to line up to (almost certainly) get knocked back, buy the visa and then rejoin the line. The line can be punishingly long.

Insult to injury, the short term visa is no more: only the 30-day OMR20 version. That's around EUR45.

Do you really think the line would be so punishingly long at 10h30pm? I'm willing to give this a try and hopefully get a friendly immigration officer or be sent to the back of the queue when I arrive there this Thursday. I'll report back anyway.

The line wasn't punishingly long at MCT tonight, but the friendly enough immigration official told me to go and get a visa. It was OMR20 or EUR60, not cheap for a short visit. I'm not sure the free visa if arriving from Dubai thing is working any more.

But you didn't go through immigration in Dubai right, whereas IAN-UK entered Dubai but just didn't go to Oman directly from Dubai.

What is this 60 euro thing? 20 Omani riyals is (exactly, due to a currency peg) 52 US dollars.

I potentially have a ~7h connection in MCT. I would very much like to leave the airport and see the city, but I'm not quite sure about the visa. Would I need the 20 OMR tourist visa, or is there some sort of transit visa available? Online search led me to a free 72 hour air transit visa, but I can't confirm if that still exists.

Definitely didn't go through immigration in Dubai, we sat on the SWISS plane while the cleaners did a quick job. (As a frequent flyer aside, there was very little uptake of new pax in DXB but something must make it worthwhile for SWISS to fly a 5th Freedom on to MCT).

I thought 60 Euros at the time was far too high for the visa, but I didn't know how to fight it. I paid OMR20 for mine and my pal paid EUR60 for his. I wonder if I could take on about it when I get back to the airport in a day or two. And as I said I am pretty sure the visa exception if you're coming from the UAE has been discontinued. But I'm not 100% sure.

I think the 72-hour transit visa costs 5 OMR and it needs to be when you are on a connecting itinerary (so in practice only on Oman Air, if that is the case with you). When the 10-day visit visa cost 5 OMR there was no point.

I think it is more about, for whatever reason, the fact that they can't (or don't want to) fill an entire plane to DXB which also means they can serve 2 destinations with one aircraft.

The BA flight to MCT used to continue from AUH, but now both cities are served directly (i.e. capacity doubled)

The transfer visa exists, but you'd need an exceptionally friendly immigration official to allow this on a two ticket itinerary :D

Well, that was exactly as expected, so i hope you don't feel too disappointed. After all, you weren't within either the spirit or the letter of the rule :D

That's weird. The visa folk bill in OMR or any other sensible currency at sensible exchange rates: it's pretty straightforward. Offered me OMR20 or GBP40. They'll also take credit cards, debiting these in OMR in a fast, simple transaction.