Alright, I've been bugged by this issue, so I tried it to see if it was really impossible...
The answer is: no problem at all!. you can have as many indexhibit installs on an MT (gs) hosting plan as you want.
Here is how you should do it:
1. Go to the manage database in your control panel. Add 2 (or more) distinct databases (will be named something like dbXXXXX_yourdbdname).
2. Go in the global settings tab (still in the manage DB menu) and Add a new user (or more, but one is just fine). Then manke sure to check the PERMISSIONS and give access to that user to all the new database you created (you need to give read/write access, not just read!)
3. Make your sub domains in your domains folder via ftp (or ssh). eg: sub1.mydomain.com, sub2.mydomain.com (do not forget to also make and html folder in each of those folders)
4. In each sub-domain's html folder, upload all indexhibit's file
5. Rename the htaccess file to .htaccess and make adjustments to the defaults.php file to get modwrite on and have clean URLs (it is supported by MT, nothing special to do)
6. launch your favorite (FF, Safari, Opera) web browser and execute sub1.mydomain.com/ndxz-studio/install.php
7. Enter all the needed information for the database (do not forget that MT database server are accessed using a special url that starts by internal.xxx, check the global settings page to get your own)
8.You should have one install completed. Redo these instructions for all the other sub-domains you want to use (possibly only from step 6 and on since your probably already setup all the databases the first time).
Alright, that's it. Please let me know if I forgot anything.