While I know some exist, I'm not familiar with any actual PC dance games. Personally, I'd suggest Stepmania as it's free, and has the capability to play all DDR releases. If you have a moral objection to using the official DDR songs, there are plenty of fan made songs out there. Stepmania also supports psx/ps2/xbox dance mats with the right usb converter.
I've seen dance pads for both PIU and DDR, but I don't recall if any of them supported both. I think I recall seeing a pad with up/down/left/right as the psx dpad, and the corners as the psx thumb buttons, so if the middle was also some other button that should work for both. If you build your own, you can have as many panels as you want.
And finally.. Yes, beatmania/hiphopmania has been in MAME for a few versions now. There are 5 or 6 different mixes, each requiring a CHD. The controls are a little akward without the actual controller though.
I'm not the most knowledeable in this area, so you'll probably want to check a few more sources.
-Kevin