I haven't confirmed that, but I would imagine it would be RMS power, basically the power handling of the wire will be the gauge of the wire and the length of the run, the longer the run the less power it will be capable of handling.
Will your components be handling 150w? That is on the upper end since most setups are in the 100w RMS range. Normally I get concerned running things at their limits, but if you keep the gain down for the speakers handling then I think you would be ok.
I agree with your concern about the crossovers in the doors, but really 90% of them are mounted there for the same reasons you are coming across and for most people there is no problem. But you will run the risk of slamming the doors finding weak solder points and such in the crossovers, but you should be ok.