I recently obtained a Laser 128 system. When testing it I tried to see if a FloppyEMU would work in any 3.5 modes, but had no luck. I tried ROM versions 4.2 and 4.5 before finding out they were for a slightly newer revision of the system.
I see some other versions of the system ROM online, mainly 2.9, 4.x, 6.0, and 6.1. I did not see this 3.5 version anywhere. Unfortunately I don't think it supports 800k drives, just a 140K drive on S6D2 on the external port. However, I hope someone with a Laser 128 on version 2.9 or 3.x prior to 3.5 can give this a try.
27256 ROM DUMP:LASER 128 ROM Version 3.5.bin