If you fly F or C on SQ (or have status), they use a *A priority tag that hangs on separately from the computer-printed luggage tag. I take these off and save them in my carryon. When I start my journey on a non-*A carrier, I pull one out and attach it, explaining to the agent that it is a *A priority tag for my later flights (I've had agents try to take them off to make room for their own airline priority tag before I explained). The tags work fine as priority tags for *A flights.

SQ, by the way, generally checks incoming connecting bags and attaches priority tags to the ones that are in C or F. A very nice service. I've never seen UA or LH handlers do this.

Generally you can't get interline boarding passes outside *A unless the reservations systems cooperate and the agent asks the system to check you in for the later flights.

Charles