Look at the Leapers MNT-HGR16S. I'm not sure what the concern is about the strength of the SW stock handguards. I don't own a SW AR but I can't imagine they are using fragile squirt-gun plastic for their handguards. Aren't they the same material as what everyone else uses minus the heat shield?
I would not be concerned mounting a two-point rail with any A2 handguard and any part like this you mount in a Colt A2 handguard that does have an interior heat shield, you should carefully remove the soft aluminum heat shield, mount the rail or whatever directly to the plastic using blue thread-locker on the threads and the correct amount of torque and then reinstall the aluminum liner just like it came out. The liner is very soft metal and is held in place by small tab inserts. Again - the aluminum liner is not there for any kind of support - it's soft aluminum that bends easily. There's also an air space between the heat shield and the handguard and the shield is for heat deflection only and can be damaged by mounting hardware. It will not offer support and the hardware that comes with these accessories isn't long enough to go through the handguard and liner without compressing the liner. Don't do this.
The SW handguard doesn't have the heat shield? Then it should readily accept a rail like the MNT-HGR16S that has a mounting fastener at each end and this setup should take the stresses of a vertical grip. If you remove the bottom handguard and you find the plastic seems so light and flimsy that you think a two point rail mount woujldl break it, you should get real A2 handguards ($20). But I suspect the SW A2-style handguards are fine.