Thanks to Hakan and crew for the firmware update. I just found this today and installed v2.0, so now I can play Region 2 discs with PAL conversion!
One issue with the install, though: the first time I put in a PAL disc, the front panel of the DV-400V read "576P" as the output mode, and I lost signal on my TV through HDMI ("Unsupported video format"). Apparently, one of the default settings after the update made it think I had a PAL TV and sent out native signal format.
I hooked up through only the yellow component out and saw that it was in fact generating video. Moreover, the logo screen read "Pioneer PAL". If I put in an NTSC disc it would change "modes" and read "Pioneer NTSC", and then back to "PAL" after swapping back. After trying different settings on the TV and DVD player, I walked away for a while to get actual work done.
When I came back, I decided to mess with some NTSC discs for a bit, trying different output resolutions. Unplugged the player to rewire cabling, then more messing around. Finally walked away once more, then came back and tried the PAL discs again... and it worked!
I wonder if the complete power-down might have something to do with it, but I see now that my video output settings would up on "16:9 Compressed" and "NTSC Video". I *think* it might have originally defaulted to "480p" and "Multisync", which I suppose would've given the player the option to output any format it wanted, including the native PAL 576P. (Makes sense, in hindsight!)
At any rate, I just wanted to say "thanks", and to record this experience for anyone that might follow with similar problems. Great unit, great firmware hack!