I'm not sure because I just tried and ... my Epson Perfection 640U was seen by PS7, for the first time. What did I do? Not sure... But I can try to figure the steps. I want to share this with you, specially because of Epson's poor OSX driver support.
Read the rest of the article if you'd like some additional info on TWAIN, Epson scanners, and Photoshop 7. This hint regarding TWAIN and Canon scanners may also be of interest if you're trying to get this working...
So, here it is. I purchased PS7 a week ago, installed everything in OSX 10.1.4. I was first interested in verifying if my scanner would be detected. I noted there was a TWAIN plug-in in the usual PS Plug-In folder, so I thought "That's it!". The fact is that my scanner was not even listed in the "Import" options :(
Some weeks ago, Epson announced a trial version of its TWAIN plug-in for X (Epson Scan to File), which installed the version 5.54A Trial in Library -> Application Support -> TWAIN -> TWAIN Data Sources. The Epson app worked well, opening the TWAIN driver and seeing my scanner. So, I was thinking PS7 would do so, too. But no chance with PS7 :(
Well, I know the excellent shareware VueScan does the job, so does Photoshop 6 in Classic mode, so that was not the end of the world. But I couldn't accept that PS7 didn't see my scanner, now that I had this new X TWAIN driver. Today, at www.versiontracker.com, I saw a post about a new unsupported driver for the SS2500/SC1200 Epson scanner & printer. As I had nothing to lose, I downloaded it and installed it. But I didn't try it immediatly. In fact, as I was reading the MacNN forums, I saw another post called "Canon Scanners and OSX" in the OSX forum. Inside there was a link to some Canon ScanGear drivers for OSX, so I downloaded them. And I installed them too!
After the restart, the first thing I did was trying PS7 and my scanner. It WORKED! My scanner was listed in the Import options, and the TWAIN driver opened smoothly, as it always did in Photoshop 6. I scannned with no problem, tried every option, all was fine. In fact, it was quite a bit faster than in OS9 and PS6.
Well, I really can't say what happend. All I can do is list the new files these drivers installed in my X Library. As we know, there are no label options in X, so my list is just a guess, I can't swear they weren't there before:
Library/CFMSupport/ESDTR_Carbon
Library/CFMSupport/TWAIN Source Manager.Shlb
The first is version 2.11, the second version 1.90b2. I'm not sure some of the files in System/Library/CFMSupport/ are not new too. But as my admin password was never asked, I guess not. That's all I can say for now, sorry. But i'm really happy: now, OS9 is almost done for me (in fact I only need OS9 for VirtualPC). I hope this helps someone.
[Editor's note: I have neither an Espon scanner nor Photoshop 7, so I can't say I've even come close to testing any of this hint! But, as the author writes, maybe this will help someone.]

