Funny you should mention it, I actually went ahead and did just that. I limited all of the font to a minimum of 15... After getting it compiled and signed, I can get to the character selection screen. Upon attempting to enter the game, a crash is encountered.
Here it is. I figure that can be fixed by imitating the updating launch method your client uses, which is what I was sort of in the process of, but it would be easier if I had the source code for your updater (hence the PM I sent about ten minutes ago). I'm running my compiled version using a modified salem.JNLP locally. The font is larger and more legible on the log in screen.
edit: I should probably clarify that I made the font increase modification to
your client, not the default client.
Here's what the size 15 buttons/tooltips look like for anyone interested:

Much less headache inducing