Widger
The mode A squawk worn by a particular aircraft has no impact upon the TCAS display. Thus an aircraft squawking 7700, 7000 or 0000 will appear exactly the same as one squawking 1066.
Gonzo
The requests you get for the use of other SIDs or radar vectors probably stem from the fact that they can be requested and granted at other airfields around the world. The "freeflow" nature of Heathrow mean that these methods don't really work at LL easily. But whilst this is blindingly obvious to an ATCO, it is not so obvious to all pilots.
On this subject, I think there is some scope for the INT director to provide headings for departures that want to start out going the wrong way to avoid a distinct cell. (eg off westerlies a heading of 120 degrees for a BPK dep to avoid a cell to the NW, then direct to BPK when clear). The coordination required at the moment is quite onerous LL INT N, INT S, SE DEPS, OCK and NE deps would all have to be involved (IIRC), but since the scenario seems to present itself fairly regularly I am sure someone could come up with a streamlined process.
G W-H