I'm like 99% sure that it is but you might want to confirm that (just check in the Map Viewer).
edit: I take it back. It looks like they're not. I could probably figure out how to do the same for regular textboxes but unless it's a huge priority, I'll get around to it later (sorry for the delay in the first place).
edit2: 'kay, I found the regular textbox "tilemap", too. However, there's a hitch: the bottom right tile has to be the flipped version of some other tile. I guess the programmers thought that since the tile would be a flipped version of the top-right tile, they could fill the space that the bottom-right tile would otherwise occupy with other data (I think it might be a palette). So the following changes (which should look very similar to the ones for the signpost textbox but with different addresses) will un-flip every tile and change the bottom right tile to a horizontally flipped version of the bottom-left tile (again, it's normally the top-right tile flipped vertically):
1. Change the byte at '0xf7328' from '0xa0' to '0x20'
2. Change the bytes at '0xf7330' from '2d 06 2d 0e' to '0e 31 00 00'
3. Change the bytes at '0xf73d8' from '01 0a 00 00 02 0a 00 00 03 0a 00 00 04 0a 00 00' to '0f 02 00 00 10 02 00 00 11 02 00 00 0e 06 00 00'
In reality, that bottom-right tile thing won't be a problem because it's just a blank, transparent tile but if you choose to expand the textbox, it might come in handy. Also, as I was writing that I realized that you could just repoint the textbox if you wanted that tile to be different, though in my opinion changing a single byte is easier and more satisfying from a "sweet, I figured it out" point-of-view. :)