I've got exactly the same problem. I tried to use NTNAME instead of USERID and it works. My server is in DMS mode too.
But I lost the desktop accessibility to the file. So I tried to use both NTMANE (for AJAX clients) and USERID for desktop development. But AJAX again requested seccond login....
Did you manage to find the solution?
I found one solution that allows for both scenario's to work. I first do a table load of my DMS users using NTNAME instead of USERID, and then I concat the following information to that table.
LOAD * INLINE [
ADMIN,MYREALNTUSERNAME,MY ADMIN GROUP
by having both the needed DMS Users and the real NT users that will be accessing the app on the file system as the NTUSER column, I now have access both ways. Does any one see any security problems with doing this?