Is there a way to "bridge" the rails on your receiver and freefloat rails to enhance stability of the handguards?  It seems like unifying the rails like this could strengthen the weapon as a whole.

Is this done?

I remember hearing about something that filled in the gap......I have no idea if it would be sturdy.

What are you trying to accomplish? Why do you want to bridge the gap? Optics??

The SIR attached to rails on the upper reciever.
I though PRI had something, and they do. But it looks like it might only work with their tubes.

Quoted: Is there a way to "bridge" the rails on your receiver and freefloat rails to enhance stability of the handguards?  It seems like unifying the rails like this could strengthen the weapon as a whole. Is this done?

Some things to keep in mind:
Bridging the gap with optics is usually not a good idea.  Using a bridge on top of both the upper and the rail will raise your sight height.
It is usually done by rails that have a bridge built into them like the CAS-V, SIR, and RASII.
With some systems, the slots on the upper may not be timed correctly with those on the rail.  I've seen this with YHM freefloat tubes.
If you buy a quality rail system, it won't need strengthening.

You could get an SPR rail from YHM or Armalite.  Armalite has the midlength rail on sale.  It is basically a riser that attaches to the entire length of the receiver as well as the full length of the top rail on a mid length railed hand guard.

is there any specific rail you were looking at using? there are a couple different companies that will bridge the gap straight off.
A.R.M.S SIR system
i do believe a couple of the daniel defense rails will bridge it too.