I post this in case someone some day will be in a similar situation as I am; that is wanting to make minimaps for their tile mdls but not having 3dsMax and the Neverblender's minimap creation toolkit (link) not working properly for them.
Why cheap and dirty? Well, you only need free software, but you also have to get your hands a little dirty and do some manual editing (as opposed to just using one of those cool 3dsMax scripts that do all the work for you).
All you need is NWN Explorer (link) and GIMP (link).
1. Open NWN Explorer
2. Go to File --> Options
3. Check the "View models in orthographic projection" box on the bottom left
4. Set a value for the orthographic view size (m)
This value depends on what size you want your minimaps to be.
16 x 16 = 320m
32 x 32 = 160m
64 x 64 = 80m
128 x 128 = 40m
256 x 256 = 20m
I use 32 x 32 minimaps, but to get a slightly more detailed map, I set the size to 20m (256 x 256) and rescale the image later in GIMP to 32 x 32.
5. Click ok
6. Click on Open, select your mdl file (or the hak it is in) and click Open again. Make sure the explorer window is maximized.
I recommend putting the mdls you want to make minimaps of in a hak file (with all the required non-vanilla NWN textures if any). This will make it easier a little later on since this way your mdl will show up (almost) in the center of the screen (because of the hak structure panel on the left side). It's also a little more convenient selecting mdls this way when creating more than just one minimap.
7. Hit the print key on your keyboard to make a screenshot
8. Open GIMP
9. Go to File --> New
10. Set the image size according to the size you choose before.
You can make templates for all relevant sizes to speed up the process when you have to create large numbers of minimaps
11. Click ok
12. Click on Edit --> Paste (or hit Ctrl+v)
13. The map image should show up almost in the center (a little down to the left of it for me). You might have to drag your pasted screenshot a little around to make it fit exactly.
13.a (optional) If you want to get really fancy, draw a blue line where (if any) door nodes are on your tile
13.b (optional) Click on Image --> Scale Image if you need/want to size it down and set to appropriate size
14. Go to File --> Export As
15. Enter the name of your minimap (standard is mi_name-of-your-tile.tga, for example mi_bdh04_c12_02.tga). Don't forget to change the file extension to .tga
16. Click on Export and then on Export again
17. You're done!
Maybe someone could modify NWN Explorer so that it exports minimap icons itself, but until then, the above method works pretty decently and is even quite fast once you get into the workflow.