That is not possible at this time. However, a menu could be written to simply write the user's choice to /config/iso_filename.txt.
There are some changes coming to the firmware which will make isosel much more flexible, however.
The reason isosel.bin cannot be loaded as you describe is it communicates with isostick by reading patterns of sectors from the emulated floppy drive provided by the BIOS when booting from an El Torito-compliant ISO image. It uses the BIOS INT13h "Read Sectors from Drive" function for this, so it must be running from a real or emulated floppy drive which can be read via INT13h calls. Additionally, since it is possible other software may happen to read the same patterns of sectors -- which cause isostick firmware to return data other than what is on the ISO, to facilitate isosel functionality -- the isostick stops listening for these patterns once it "looks like" the host has passed the boot stage or is within an OS.
There is, however, a to-do entry for documenting the comms between isostick and isosel. I would like to make isostick more flexible for third-party development and hacks, so when those documents are completed, isosel will likely be open-sourced as well.