First, Considering that W98_slip has lain fallow on my hard drive for over a year now, I doubt I will ever have the time or inclination to adapt it for an OS I don't own. That said, since I don't own ME, I have no idea of what problems would arise from trying to apply this slipstreaming method to it. If ME's setup files are digitally signed, for instance, then you're probably SOL.
THe place to start in adapting w98_slip to work with ME would be to look at my first follow up to the original post in this thread, in which I described some of the ins and outs of SE's cab files. Read up on Cabinet files and makecab (the Cabinet SDK documentation isn't the best but it's still the place to start) so what I said in that post makes sense, and then experiment with ME's setup files and see if they behave the same way as SE's file do or not. The size of the cabs doesn't matter so much as whether they are linked in a chain or not. If ME's cabs are not linked in a chain the way SE's are, then you'll have to do significant extra modifications to make W98_slip work for WinME.
Assuming that the ME cab files are chained together, then making it work with ME shouldn't be too terribly hard; the main thing you'll have to do is modify the scripts that generate the makecab directive file so that they create cabs that break at the breakpoints you find in ME rather than the breakpoints found in SE.
The hard part, of course, is finding all the little quirks that need to be addressed. For instance, a short file name in SE contains a parenthesis, IIRC, which is a character that Minitrue treats as special; the entry for that file in the file list had to be specially handled to take out the parenthesis before I could use Minitrue to process the entire file list. ME may or may not have similarly named files, but you can be quite sure it has something quirky.
W98_slip is not nearly as hard to hack as, say, HFslip; as batch files go, it's pretty basic, and the hardest part for me was learning to use Minitrue to do the donkey work of manipulating file lists into makecab directive files and layout*.inf files. That will be the hard part for you, too -- fortunately, minitrue's documentation is fairly complete and well-written, and you'll have my original minitrue scripts to crib from.