/etc/profile didn't get updated on my N900 with the previous version either, I assumed it was something unknown preventing that so I didn't bother with it - I edited it manually into my local .profile anyway. But the postinstall script did run though. Did you verify that the 'x'-bit was still set after editing the file?
Also, with the symlinks, be careful to either use relative symlinks, or, if you create absolute symlinks, that they are pointing correctly (because 'tmp' and all that won't be there after the install of course)

It sounds like you're nearly there - as I said above the only thing I can think of would be a missinge execute bit on the postinstall script. After installation you can always check with ls -l /var/lib/dpkg/info/maemotex.postinstall

edit:
Two more things:
1: Most packages use 'postinst', not 'postinstall' - I don't know if it makes any difference.
2: To do stuff when you un-install a package (like re-updating /etc/profile) you'll have to add a postrm script as well.