Use the "Disk Drives" pulldown item instead of the "Open image" or "Boot image" item.
Set the mode to "R/W" for that drive before opening the image with the "..." button. That should do what you want and without reading the entire image first.
As for the "experience" - I suspect it'll be a bit underwhelming. A standard HD 3.5" floppy is formatted with 18 x 512 byte sectors per track, and double sided makes that 36 sectors or 18K per cylinder.
So, an 88K 810 image will occupy about 2.5 cylinders, a 130K 1050 image will occupy about 3.5 cylinders (vs 40 tracks on a real drive).
But potentially you could use large ATR images which use the entire floppy. You could also have multiple virtual drives using images on 1 floppy though it'd cause slowdown. The actual speed - from memory PC floppies run about 20K or maybe a bit more per second so a disk image could be read fairly quickly. Also, it'll probably be cached so subsequent reads might be just as fast as a HDD based image.
On linux at least, you can set weird formats as you like. Lots of controllers don't support FM, but you can probably convince them to do 130k mode, by telling it you have a 40 track drive that is single sided, and to use 128 byte sectors, and to use 26 sectors per track.
Of course, you need a *real* floppy controller for that, USB floppies aren't going to cut it.
Also, I bet there's a way to disable the caching in linux, I just don't know it.
Windows? you're screwed regardless, without writing new software, and possibly even a special driver to do the atari formats.
Edited by Joey Z, Tue Apr 25, 2017 10:35 AM.