hmm, i do not know... i had half life years ago, played only 1. stage, did not liked much. So i have no possibility compare mdl files for now, but there is may be something like pin on steam, but if you have gog version, can be problem. Or authors of rebel galaxy did own format mdl.
well, i can set options for half life and try.

edit:
using crowbar is pinned to steam

ERROR: The game's model viewer, "C:\Program Files (x86)\Steam\steamapps\common\Half-Life 2\bin\hlmv.exe", does not exist.
Possible causes: The game's SDK or Authoring Tools has not been installed (usually via Steam Library Tools) or the path given to Crowbar (via Set Up Games button) is incorrect.

i do not use steam games, i prefer gog, so this program i can exclude.

could you give me a sample .mdl file of yours? I can try to get some more info about this.

if you diged into rebel galaxy .pak file, you know this ship :)
https://ulozto.cz/!0jMxROf2gXwD/human-frigate-zip

A Quick look with a hex editor seems that the .mdl files used by rebel galaxy are Unity 3D "Mesh Serializer 2" 3d models.

It seems to be an Open Source format, and some insightful info can be gathered here: http://wiki.unity3d.com/index.php?title=MeshSerializer2

Thanks, i did see this info in mdl, but i little underestimated it and had hiden hopes made things by easy way. I have not experience with unity yet, though i found similar files in SOSE, simply there was not urgent need work with such format. Yesterday i got idea check one of models, so my amateurs troubles started. :)

i did some experiments with models from rebel galaxy
https://ulozto.cz/!Z1HsNCS5QNHa/tennhausen-jpg
https://ulozto.cz/!a7DJvFKXFUPi/tennhausen2-jpg

Thats awesome, how did you do it?

i myself rather dont know :)
It looks, that model has 2 or more bodies opticaly as one with different textures layers, you can see it in my zip, where you reckognized unity. I observed some kind dynamic change of appearing of others textures, when ship is damaged or wheathered, but diffuse (main) textures are somehow ripped for individual bodies. Fortunately, most of texture is on one body and small part on second, so there just must be defined which is right for our purposes (i did it visualy in game).
For example starting corvette has 3 bodies covered in one, tennhausen has 2 bodies, ...
i was still unable made progress with unity, so i used old program 3d ripper, but this works on 32 bits only.
extract from ripper (many objects captured) import into 3dmax8, here is important define body, which we need, rest can be deleted (practice). then i exported as obj (as imd was not working) and open imported in blender. we can see dual or triple body. can be used some trick for reckognize these parts in game environment. For example dual is set as paralel bodies, one part is changed(size), then export as obj. Imdeditor convert it into imd + set adequate texture. in game we can decide, which part is "right" and in editor (3dmax or blender) we can delete other, then resizing, setting orientation adequate for x,y,z axis, etc...
For start I did small test with ripper in SOSE, there is just few objects from start of game and we can give command for destroying itself.
I believe, that right learning for unity can speed up whole process.

For first aproach could be enough, i am still in first stage of it.
There is test for SOSE
https://ulozto.cz/!8wuaezldelLc/cargo-jpg

I remember EVE Online models also had an similiar approach to apply the textures to the models.

I think im also going to try some things to get the .mdl to work.

After some further digging around in the PAK file, I have successfuly extracted every model in the game, but sadly with no way to open them.

Even though they supposedly are MeshSerializer.1.41 Models, the developers have most likely changed the format a bit.
I think the only way to get all the Models from the game now is 3D Ripper, which would take quite a while to get all the ships.

If you are still eager to get some proper help on the custom MeshSerializer Format (namely "MeshSerializer_Runic") you should ask for help either at Xentax or ZenHax.
Im sure the guys over there would be way more effective in getting this format reverse engineered and exported.