Will there be the option on the V25 to supply your own regulated +5v (I have a Belleson reg based PSU I'd like to use).
There's no deliberate option to bring in an external source of 5 V. That said, you could achieve this by removing the U11 5V regulator entirely and connecting a +5V and Gnd at the solder pads. The existing regulator uses the 7805 pin out although it's not a 7805.
Before going down that road it's worth reviewing the power supply arrangement and asking yourself if this is really worth it.
First, the existing U11 5V regulator is a high performance low noise switching regulator that's essentially a 7805 drop-in replacement. Second, the 5V supply is not directly used to power the primary LDRs although it is used to power the on/off LDRs used for input switching. Lastly, the 5V is further regulated down to 3.3V via a low noise low dropout linear regulator. The 3.3V power is used to power the processor and to regulate the LDRs used for volume control.
In my view, the V25 is unlikely to benefit from being powered directly from an external 5 V source. I can't prove that. It's simply an engineering judgement. I've been wrong before about the impact of power supply quality on the V2 board design but the V25 is quite different from the V2. With the V2 the incoming power source is used directly (no filtering/conditioning etc.) to power the 2 op amp chips that are part of the LDR control circuit. With the V25 there's 2 stages of voltage regulation (SMPS followed by linear) upstream of the control op amps.