I only did some basic stuff with the DVR-810H software, and never got around to messing with the SD-400 software (I was so disappointed with the internals that I never got around to it). The burning question is whether you can hack them, and the answer is yes. I don't have the tools to do the prom hack myself on these units, but I was able to do monte on them just fine. As with all s2 units, you can use the 3.1.u5 kernel and minimal filesystem from an hdvr2 as the basis for doing monte. tivoftpd and bash worked fine, tivoweb worked fine after the standard tweaks needed for 4.0 units (and maybe one more, its been a few days). I didn't have a chance to test extraction, but you should be able to extract by doing the same type of kernel and tivoapp patches that are needed on 4.0.
Expanding the hard drive on these units has apparently proved to be a bit tricky. One nice thing about the 5.2 software is that LBA 48 was included, due to the newer version of the linux kernel which was used. My understanding is that expansion of the drive space on the SD-400 and the DVR-810H software is problematic. You can definitely use standard tools on the DVR-57H software, and you can use the DVR-57H software on the DVR-810H. I didn't test the DVR-57H software on the SD-400, though I wouldn't be surprised if it worked. Because of how the units are designed, you should still be able to use two drives (assuming they will fit in the case) on the primary IDE channel. It is unclear if anyone has attempted a 3-drive setup on the Pioneer units, with two drives on the primary IDE channel, and a third hard disk on the secondary channel with the DVD drive. The PTVupgrade guys have apparently been able to use drives on either IDE channel though.
-More to come, stay tuned-