Like you, I thought I was SOL on using a RAM disk for various speed-up operations. It's not totally implemented in OSX, but I have found a way to make it work for *some* applications by running it in OS9.1 (Classic). Here's how I did it...
I use the shareware app Rambuctious, and make a RAM disk to whatever size I need. For the sake of example, let's say the 12MG I use for my Netscape image Cache Folder. I then put it (or an alias of it) in the startup items folder of OS9. Then when I start Netscape, I use a profile that has the preference set to use the RAM disk. It works fine. However... SETI@home no longer finds the RAM disk in OS9 like it used to, so I'm not sure exactly what's up with why it works with some apps and not with others any longer. The upside is that I can get it to work by running Classic (which is only a temporary workaround). The downside is I can't get it to work for all apps... I hope that answers your question to some extent. If anyone out there has any other experience/knowledge of this issue, by all means please pipe up!!