Total HYD failure would still leave reversers available through the standby system (though deployment times increase), along with accumulator braking, availability of which, as usual, is governed by the number of brake applications rather than braking force reqd. I think anti-skid is still available as well. So, RTO ought to be possible but not necessarily the best idea - it depends on a lot of things, as discussed above, and as it ought to be be discussed in the de-brief.
Interesting to note that Mr Boeing has covered himself well - all the associated QRH procedures and performance data (at least in our outfit) are referenced to LOSS of System A (or B), not any particular caution light - assuming, once again, that that has been correctly diagnosed as the actual condition of the aircraft. The Low Pressure lights are just one clue to what is happening.