After looking at the code, the tw and th (widthmult and heightmult) values for the resulting texture created with TextToTexture are taken from the original loaded texture, they're simply copied over.
This code loads the "Open" button with horizontally-flipped text.
Load data/black.jpg, BlackFlipped, -1, 1
AddText BlackFlipped, ButtonOpen, nimbus_sans.ttf, 28, Open, -1, -1, -1, -1, center, center, 255, 255, 255
The only real difference there is that the Load command uses the -1 value for "th", and I used a new name for that texture. The "AddText" command reads the "-1, 1" value from the original texture, creates a new texture, applies the text, and applies the same "-1, 1" to that texture.