When I was in university we played many, many games using such contracts.
We didn't use markers for the contracts except in a few extremely complicated cases, and we probably contracted just about everything, including robber placement, futures markets, and building patterns, e.g.
- "I'll give you a wheat to not put the robber on me." "Oh yeah, I'll give you a wheat and a wood to put it on him!", etc.
- "I'll build my settlement here (leaving you the third space on this lucrative tile), if you don't cut off my road over there."
- When we played with Cities and Knights, lots of resources got tossed around by people try to cause or avoid the barbarian invasion.
We did retain the rule you could only trade on your turn, so you had to wait for either person's turn to finish the contract.
We also, rarely, reneged on contracts. The most common reason for reneging was that you could win that turn if you didn't honor it.