This has been an ongoing issue, and I just realized I have the answer in my hand. JakeJP's Mms.apk has already been merged with 2.2 Froyo based CM-6. This has built-in user agent spoofing allowing people to use MMS with softbank. Of course I cannot confirm that it works, or give more detailed information on setting it up because I don't use softbank.
Screenshots from 8/10/2010 nightly build of CM-6. Change the user agent to iPhone (3.0)
If you are running CM-5, replace your current Mms.apk with JakeJP's Eclair version (latest version: Mms.apk.jakeMod11) or upgrade to CM-6, which is currently at release candidate 2 (RC2), which for me was extremely stable. So are the nightly builds, which already include Piaf's extremely cool, tweaked powerstrip widget. Directions for installing CyanogenMod are at the wiki.
Jake has info for eclair and froyo on his personal site (in Japanese).