I do not know if your information is correct or not, but the spread [distance] between the legs on a MFT 800 is less than on a MFT 1080. You can make your own diagonal brace stabilizer. If you disassemble your MFT, you can insert internally threaded nuts into the bottom channels for attachment of braces and other accessories. 10mm square nuts and the elongated nuts that Festool uses for the side rail stops which are threaded to accept 6mm bolts and screws, fit perfectly in those channels. Festool uses 10mm square nuts to fasten the cast corner fittings.
A brace can be made of a length of wood and a pair of strap hinges. One hinge is connected to the underside of the MFT side rails using those nuts as anchors. The straps of the other strap hinge are bent to enable them to wrap around the cross member of one of the MFT pairs of legs. Alternatively, a shaped wooden bottom end clamp can be made of plywood and secured the the brace rod with a bolt and hand knob.
I have not tried it, but I think those telescoping painter's poles could be modified to work as a lightweight, adjustable brace. The telescoping legs from a sacrificed camera tripod might be suitable, too.
Or you can buy the Festool brace.
Dave R.