Ok, so I've found out that one was "Kitahe: Pure Songs and Pictures" Which is the soundtrack to Kitahe: White Illumination. Alternate spellings are Kita e, Kita he, and just Kita.
edit: using that one as a starting point, I found information on some more. http://www.g-res.com/News/Dreamcast/199 ... 90502.html
A search for "MIL CD" on there will turn up a list of 4-5, but they are in japanese so it's not much help.
edit 2: and some more: "HANG THE DJ" MJCYX-80002
"HEARTBREAK DIARY /deeps" PICL-7501
and apparently the code for white illumination is ZMCZ-656
http://game.musicrelease.info/search/na ... sort=score
edit 3: AHA! SWAT's site had a few in a list:
SCLX-2004/Snappers/Nine Chairs/-/SENHA & Co.(1999/6/25/MIL/2415//YES)
ZMCZ-656/KITAHE(Go North) White Illumination PURE SONG and PICTURES/-/Media Factory(1999/6/25/MIL/3200//-)
PICL-7501/DPS HEARTBREAK DIARY/-/Pioneer LDC(1999/7/22/MIL/1200//YES)
FSCA10113/D2 Original Sound Track/-/First Smile(1999/12/17/MIL/3333//YES)
MJCYX-80002/HANG THE DJ/-/Marvelous Entertainment(1999/9/17/MIL/1500//YES)
WPCV10045/HIMITSU Original Sound Track/-/Warner Music(1999/10/14/MIL/2913//YES)
That makes a total of 7.
edit 4: Full name of the Checkicco disc is "Checkicco: CHECKICCO NO MIRU CD"