At present, the only way I'd been able to get it to not freeze, is if I intentionally disable the modchip in my PS2 console and then hit reset to let it boot normally.Did you get it to work, or does it still freeze? I had the same issue as you did (i.e. my PS2 froze when booting to OSDSYS with FHDB installed), but that didn't happen on some of the older FMCB installers (I tried specifically v1.964 and v1.94, both of which worked). However, now it boots to black screen, but I guess it is progress either way.
As for HDDOSD: It seems you can install it with a hacked version of the Sony Utility Disc (tutorial in french here), which is available if you google SUDC3 (as in Sony Utility Disc Compilation 3. Cannot link to it, as it is copyrighted material). I haven't tried it myself yet, as you need to burn it to a disc and run it on your PS2 (it doesn't boot from OPL, apparently), and I don't have any burnable media lying around. Maybe this information is useful to you
At least with some help with from @Ryccardo I was able to get HDDOSD working, so that's a step forward. Using HDLoader ELF is rather slow for ripping, but now I notice that when looking at the HDD data, it actually shows the games on there. I'll have to look up HDLgameinstaller and see how that gets on.
Question: Where should I put ELF files for apps at on the HDD to load them? I Tried putting the HDLoader elf on the hard drive but it never loads from there. Also noticed how fussy the PS2 is for which USB drives it will load from. For example have an older 128MB USB drive I was going to use but it refuses to load any ELF's from there. but then loads from my 1GB Lexar without any complaints.