I am a scientist at a pretty reputable institution. I have some simple research ideas that I do not have time to develop myself. From my past career, I am in good contact with some university professors. Is it acceptable to ask them if they are willing to discuss such ideas and eventually open a master thesis project on the topic of which I will be the external supervisor? I think this could be a win-win situation for everybody, at the same time, I wonder if this will be considered rude.
I don't know what reasons could exist that would make this not ok. In fact, things like this happen all the time already: external research institutes, companies or other kinds of institutions offer master or bachelor thesis positions that students can apply for. And in all cases, the main supervisor is still a professor from a university (as this is required by the universities).
Often, these positons are not even discussed with universities/professors beforehand, but simply created and once a student has been found that is interested in this position, they will bring the topic to their professor(s) to find someone willing to be the supervisor on that topic. As long as the topic is relevant for the studies, has enough substance and novelty to be a Masters/Bachelors thesis subject and fits within the area of expertise of the professor, it is normally never a problem to find a professor willing to supervise.
(I am based in Germany, it could be that things differ in other countries)