Yes, I'm sure the ReeCharge will be fine for the 705 too. I've been eyeing it up on the Evans site, hovering over the buy button.
I've been running the 705 since it first came out, and can't say I've ever had any issues powering/recharging from an external battery, regardless of the software version - I'm pretty sure it was V2.20 when I got mine, the oldest file I have on Garmin Connect is from sw V 2.40 (back in 1998). As long as it has 5V into the USB port (with a small caveat about connecting the data pins together, which the ReeCharge should do, to stop it going into the mode where it thinks it is connected to a computer) then it will both operate and charge simultaneously.
I'm slightly hesitating as that price would buy me another couple of portapower batteries. Three of those would be more than enough for PBP for the phone and GPS, and have neater cabling on the bike. But being fully self-sufficient power-wise also has its attractions.