Any hints on what O/P Plats might look forward to

regarding benefits on US or UA after 27 October

Probably the same thing as OP Gold....Intl lounge access.

What about baggage allowance

bonus miles and preferred seating

From the Star Alliance Website:

Star Alliance Gold Benefits
Priority Reservations Waitlisting1 - when there aren't any seats left on your preferred flight, gives you priority should a seat become available.
Priority Airport Standby - gives you top priority when standing-by a flight at the airport.1
Priority Airport Check-in - allows you to check-in at the Business and/or First Class counter
Priority Baggage Handling - like you, your bags get priority treatment and are among the first to be unloaded.
Extra Baggage Allowance - an additional 20kg (44 pounds) or one additional piece of luggage2 which means you can check in three bags instead of two.
Priority Boarding3 - enjoy the freedom of boarding at your convenience along with First and Business Class passengers.
Airport Lounge Access - available worldwide for you and a friend4 when you travel with any Star Alliance member airline, regardless of your class of travel.

http://www.staralliance.com/en/benef...silver-status/

Lounge Access is available for domestic travel as well, right?

Excludes UA and US clubs, so expect it to exclude CO clubs as well.

Unlikely if you have a CO-issued *G status, just like US and UA exclude it.

Not quite. It is excluded if you have US or UA-based *G credentials. I have other *G credentials and access the RCC as much as I want on domestic itineraries.

Well, not quite. It's excluded for US/UA clubs with US/UA *G credentials. Other clubs are fine wiht US/UA credentials, and as you point out, US/UA clubs are fine with non-US/UA credentials.

As a CO Gold with a Euro address, will I be able to access PC on purely domestic itineraries? Or is this just for foreign programs?

If it works like US and UA, then it is the program that excluded, not the address. My guess is that it works like UA and US.

But... CO has given PC memberships to European Golds and Plats for the past few years. This was a full PC membership, with a separate card and full privileges. If I had to guess, I would think this is going away, but you can keep your fingers crossed.

From the CO website:

Star Alliance Gold Elite members may also access Presidents Club lounges and designated Star Alliance Gold lounges. Continental, United and US Airways Star Alliance Gold Elite members may access any Presidents Club, Red Carpet Club and US Airways Club on the day of international travel. All other Star Alliance Gold Elite members may visit any designated Star Alliance Gold lounge before departing on a domestic or international Star Alliance member airline flight.

http://www.continental.com/web/en-US...ralliance.aspx

So, *G members can visit an RCC club with a domestic ticket?

Please redirect your question in the specific forum set up for this.

NO.

Read the WHOLE quote there. Or read one of the many times it has been asked in the other thread (http://www.flyertalk.com/forum/conti...-concerns.html).