The 757s will be transferred on T5 day to the Spanish routes which are not moving to T5, but going across to T3, as the aircraft have bulk baggage loading and T5 will be containerised only. The facilities in T3 will ultimately be shared with the codeshared Iberia flights, and indeed with all other OneWorld carriers (eg Qantas, Finnair, Malev) who will move around from their current terminals. Yes, it is an inconvenience there is little publicity about. Presumably these BA flights will come over in 2012 when the remaining satellite at T5 is completed (and the 757 will be out of the fleet).
It's the same as at present where A321s only operate out of T4, not on T1 routes. BA ops may get a lot of stick, but they are quite able to handle divisions like this.
Regarding the markings at the stand positions, that is a completely different matter. All sorts of aircraft, including 757s, from all sorts of carriers are currently parked on T5 stands, and this presumably will continue to some extent, the pax being bussed to/from other terminals as at the moment.