MRT’s are not supported in the current versions of OGLES. I have seen a number of developers manage to utilise deferred shading on the 3GS at interactive frame rates, but this can only be done by working within the APIs limitations (i.e. performing additional render passes).
We cannot discuss the capabilities of the GPU in the 3GS.
Yes, PowerVR is a TBDR architecture (see “SGX Architecture Guide for Developers” on our documentation page for more information). There are currently no mechanisms exposed for directly accessing on chip memory.