You boys are re-inventing the wheel here...this stuff has been going on for years out here in the PNW.
Crossbow bolts and long bow arrows made specifically for line placement are available from New Tribe. They are fiberglass, with a blunt rubber tip, heavy enough to pull 20 or 30 lb. test fishing line down. Mounts for reels are both available commecially, or are home-built. Sling shots are commonly used out here in my part of the word, and New Tribe sells special tiny throw bags to use with sling shots, too.
To me, the use of crossbows and long bows only make sense when TIPs are somewhere over 100 feet. The slingshot works anywhere a Bigshot does, but doesn't have as much range. The one I built is good for about 100 feet. It's compactness means it is superlative when one needs to advance a line in the tree or place a line from one tree to another.
It is amazing how much having a line attached to a projectile cuts down on the range achievable. If you really put Black Widow line on a crossbow bolt, it wouldn't go much of anywhere...there isn't enough mass to the projectile to carry the weight. Even Zingit imparts enough drag to cause problems. Monofilament or braided fishing line on a reel is the only really successful arrangement.
I think for arborists, throwbags and the Bigshot are the tools best suited to their environment. Only big tree climbers in unpopulated areas will find bows a logical choice. Slingshots might fall in between. Of course, the Bigshot can play in both arenas.