In the trenches in WW1, a sling might have been useful. I could imagine a soldier putting a grenade into a sling and throwing it over the top at a target. It would dramatically increase the range of the grenade.

Did anything like that happen?

They did bernardz, we already discussed this topic.
Try to do a research using the "SEARCH" button or both the Indexes here http://slinging.org/forum/YaBB.pl?num=1345914175, and please continue one of the existing topics if you need to  :)

EDIT: this morning I feel so altruist that I made the research for you. Here're the topics about WW1 slinging:
http://slinging.org/forum/YaBB.pl?num=1263499457/14#14
http://slinging.org/forum/YaBB.pl?num=1176944922/7#7
Search "slinging grenades" and you'll get much more topics, referring to the Spanish Civil War as well.
Now I'm locking this topic  :)
Greetings,
Mauro.