It appears you may be able to engrave the slide as an alternative location, but it appears you need approval to do this.

Here is some info from the Texas Law Shield forum:

"In regards to other locations to be stamped: If there is not enough room, you may have to request a variance. As stated above, each maker of a firearm “must legibly identify it by engraving, stamping (impressing), or otherwise conspicuously placing on the firearm’s frame or receiver” the serial number and additional information listed above. A variance can be requested by submitting a letter to ATF’s Firearm Technology Branch. Please see § 7.4.4 of the NFA handbook, or call TLS and ask to speak to an attorney for details on the content of the letter. "