I think it's the nature of a jam that "it will be what it will be". I'm not sure you can shepherd people into playing the kind of music you want them to so much. Some might be receptive to a few pointers (I would!) but I suspect it's more that you'll attract some of that ilk, and others who aren't that way inclined just won't show up.
I help run a jam night once every few weeks (full band setup). A lot of the jams in the area suffer from the jammer's safety-net of endless blues songs. I quite like blues, but not 6 songs in a row that all sound the same, just because no-one could think of anything else to do.
When my band host the jam we start off with 3 or 4 tunes just to lay the ground and check the gear. We play rock and groovy pop, and not much blues. For some reason this seems to influence the kind of people or kind of music played at our jam nights. It's quite varied and we see precious little of the blues stuff. OR if there is, it's a deliberate thing and not "the default option", which is quite a relief & nice to hear. Other people run jams in the same place, on other weeks, and they seem to have a different slant depending on who'se hosting.
This is all by accident: we don't deliberately try to influence what kind of music to play - anything goes!
I've tried to work out why what we play at the start SEEMS to influence what happens during the jam. My conclusion is :
What we play at the start sets the scene and a mood. It goes quite nicely and I hope we make people feel very welcome, so I guess people feel "invited" to do something of that ilk. In that sense we've laid the ground, and others join in afterwards.
We generally stick about as 'spare musicians' in case someone wants to do a specific song or we need another bassist etc. That means for such people my band's repertoire influences what happens
The alternative is you could be more direct & bill it as a "Jazz jam" meaning you'll immedately attract jazz musicians who are inclined to jam. That would lay down an expectation.
We just advertise as "electric jam" because really anything goes.
So my thinking is that just the nature of the kind of thing you play (with your band at the start ?) will influence who does/doesn't arrive, and if you bill it as a Jazz Jam (or whatever seems appropriate) that'll set the tone right away.