I was looking into code and it does not look too hard. Basically, skip (*i)->removeResearch(*iter); in GeoscapState.cpp when "repeatable" flag is set, and do processCommand for missionTriggered param if one is present.
What possibilities would this open. It would allow "buying" missions for highly configurable price in Brainer's and Runt's time and items. And chain missions in different ways, with forking, even some C&C, etc. It would make Brainers useful even if you have run out of all (or all interesting) researches.
~50 brainerdays seems a reasonable price for something that you should not use lightly, and can be way more for late-game missions. Including some kind of prisoner in price could be a good idea too, as they are harder to keep for a long time, and it can give use for many prisoner types.
For example, instead of waiting while RNG would give you a Reticulan Base Probe, you would craft 10 SG novices and 10 Data Disks into a "Lead to Reticulan Base" item, research it for 300 Brainerdays, and get a Reticulan Base mission on finish. With 3 Guild Reps and a hefty sum of money you will be able to research Guildmaster Location. Or it could be something even more complex, detective-like work, with combination of questioning different people, checking different leads, etc. It can be useful for other mods, such as X-Com Files.