Did you ever find a solution? I am also trying to do this.
use two fields enstead. start with field - type number and on the first letter, copy the content to another field - type text and select the second field or use field - type number as temporary field for number and text input and copy the content on object exit to another field.
Surely there has to be a way. I'll continue my search...
In the same situation I had to create my own keyboard
not such a big work
the same script for all keys with different script parameter
For this solution I have specific keyboard for integer, decimal, and letters... my users love them
more explaination in this thread :