Its hard to say since the trans was under a load pulling a trailer,. but on my 49, I blew that trans cooler line off twice and ran it out of fluid about 3 months apart (I've redone the lines this last time) and it still works. Both times for me the trans was cold, having just started it after it had sat for several hours.
Once the fluid level get below the filter, the trans quits functioning, and that happens pretty fast when a cooler line blows. Once the fluid flow stops, all the internal pressures go to zero instantly. The only damage that can happen is during the time between the full fluid and no fluid, maybe a few seconds. I would think mostly the hot parts loosing the fluid flow are mostly what would be damaged, but as soon as the pressure drops, nothing should be under load.

The cooler line has to be repaired either direction. if you take that expense away, how much dos it cost to take the chance? It does have a lot of miles on it, but do you feel comfortable with the place its at rebuilding or replacing the trans?

Fix the line, and dump $100 of fluid in it might work for a long time, or it might get you someplace you know and trust better, or you may have wasted $100. If your really concerned, forget the filter change (if it adds that much to the cost of taking a chance) you can do that at a different time. I suspect you have probably wasted $100 before on something. Put it in proper perspective.