hmmm,
I imagine that milling on a Jacobs taper is going to be worse than milling on a Morse taper. Not only will you stand a good chance of damaging the bearings, you will stand a good chance of ruining the taper.
So, if this was my only option, I would make an adaptor that would only hold one size on mill shank.
Take a short rod of something like 3/4" or 1" round rod. Square up the ends as best you can on the bench by hand. Clamp it vertically, centered under the quill and spot drill. Drill what ever size shank you will be using (make it 1/4" or less - that is a JT 1 after all), follow with a drill the size of the small end of the taper, follow with a JT 1 reamer, follow with a drill and tap for the locking threads (use a plug followed by a bottoming tap). You might want to see what it costs to have a local shop make it, it would be a quick simple run for a CNC lathe. There might even be someone here that can do it - tho the JT 1 is an oddball. Modelers tend to use JT 0, and the rest of use something bigger.
Now you have a tool that will screw up onto the threads, center on the taper, and have a concentric hole to hold a mill. Just add a pair of grub screws to secure the mill and take light cuts.
Unless you get a good price on the drill, tap, and reamer, you might not be too far off the cost of a new mini-mill that will do a better job.