So, this thread reminded me that about ~2, maybe 3 now, years ago I made a sort of layout for a spar challenge system I was thinking about creating, or wanted to be created (I wasn't exactly a developer from what I remember so I just mapped it out).
Anyways, I decided to look in the weapons list and sure enough it's still there.
So basically, you decide you want to challenge a player to a regular spar or a bet spar. If you're inside the spar complex, simply right click them and you'll see this option:
You click it and then this pops up
(Those two radio buttons are supposed to be regular/bet spar, didn't look into why that didn't load and just took a picture to show the GUI)
It's pretty simple, you just pick the options you want to set. As you can see, I added quite a bit of options for players.
Anyways, I completely forgot about this even existing until just a few minutes ago after I was scrolling through this thread. I'd be happy to complete it as soon as I have time after finishing up a few other projects I'm working on, it seems like it'd be a fun project, if you guys are interested in seeing it happen.
If not then it can give you guys a sort of idea of how to make/layout whatever spar ideas you've been discussing.