It's possible after commit 3630506. Look at src/stand/lua/gfx-orb.lua and src/stand/images/freebsd-logo-rev.png.
It replaces Forth. loader(8) always had a language. The whole beasty menu and everything around it is coded in Forth (hence the *.4th extension of the files).I personally feel having Lua (or even any graphical output) in the FreeBSD bootloader absolutely bizarre as a decision but it doesn't really hurt anything I suppose.
Yep, I remember having a fiddle with the .4th stuff around the 9.x/10.x release. For a brief time, the BTX loader developed an issue booting from my DVD drive if I recall. From all the customization that Forth provided, I still needed to recompile the whole thing regardless to apply the fix and make it work.