I am ticketed on PHL-DEN-SEA on 5/17, leaving PHL at 4:54pm on #306. If I decide to stand-by for an earlier flight, can I route thru ORD instead of DEN?

I understand that so long as origin/destination are the same or are "co-terminals" of your original destination, and the proposed routing is considered a "legal" routing, you should be accommodated. Sometimes, offering to connect via another city actually helps UA where wx or oversales are concerned and, if that appears to be the case, I've found that a plus when asking for such a standby routing. Of course, if you are trying for the old VDB, you might want to stick with your ticketed itinerary.

I've been told that standby for routing change is forbidden
under normal operations but have been (not consistently)
accommodated "as a favor to one of our best customers."
Of course, if you show up, like, way early, they may be
the ones to make the suggestion: seems it's okay if they
initiate the change, not if you do.

It's not really clear in the UA rules. It doesn't specifically say you can't, though:

NON-REFUNDABLE FARES
--------------------
FOR ALL TICKETS ISSUED NO MATTER DATE OF ISSUE OR TRAVEL:
-PASSENGERS HOLDING CONFIRMED RESERVATION AND TICKET MAY
STANDBY FOR ANY EARLIER/LATER SAME-DAY FLIGHTS FOR NO
ADDITIONAL CHARGE
-THIS INCLUDES AT MULTICITY AIRPORTS LISTED AS FOLLOWS
UNLESS RESTRICTED BY THE AIRPORT PROVISIONS OF THE FARE:
CHI CGX/MDW/ORD
NYC JFK/LGA
WAS DCA/IAD
-SAME-DAY STANDBY FOR ANY FLIGHT IS PERMITTED..INCLUDING
FLIGHT-SPECIFIC FARES AND CONNECTIONS STANDING BY FOR
NON-STOP FLIGHT ..AS LONG AS ORIGIN/DESTINATION AIRPORTS
REMAIN THE SAME.
-CUSTOMERS MAY BE PUT ON THE AIRPORT STANDBY LISTING
ONLY WITHIN FOUR HOURS OF THEIR DESIRED FLIGHT TIME.

A fewweeks ago I was ticketed SEA-PDX-EUG-SFO-PHX

I got to the airport early in SEA , not feeling well and then sent me SEA-SFO-PHX.

Two days later I was routed SEA-SFO-PHX and they sent me SEA-LAX-PHX. So I think the poster who said as long as origion and destination dont change its no problem was correct, never was "as one of our best customers" mentioned

I would be very hesitant to change routing unless the load level are low.

Isn't it possible to make it onto the first flight standby PHL-ORD but never make the connection on the second flight ORD-SEA if that flight is full or oversold?

You could be stuck in ORD for quite some time, whereas if you keep the original routing, you might just have to wait for the original second flight out of DEN.

Correct. It is permissable but you would be on stand-by for the connecting flight without any confirmed flight and the risk if you get stuck is all yours.

Good points about not being stranded in a connection city. I had assumed that when the first flight cleared, the passenger would have gotten a sense from UA about the downline chances, so would know what to expect. However, misconnects or irregular ops. downline can happen while the first flight is in the air and those can change the standby likelihood + or - for the connecting-city flight. Reason #683 why I'd like the return of 1K same-day changes.

This was always my understanding, but I have been denied when trying to get onto a direct flight when I was originally on a later connection (IND-DEN instead of IND-ORD-DEN). Of course, the last time I tried that was a couple years ago.