OK I think I've provisionally got this sorted - you have to edit the keymap file using a text editor (either on the device using e.g. Total Commander or (probably more easy) copy the file to your PC using Activesync, edit it then copy it back to your device). On my snap the file is \Windows\eT9.Maple.wwe.kmap.txt - might be different depending on your ROM and language settings.

Before we go any further, make a backup of the file because you can get into all sorts of problems if you mess it up. It will be recoverable with a hard reset if necessary, but you probably want to avoid that...

The process is pretty self explanatory when you look at the kmap.txt file, but as an example:

Look at the entry for the R key:

{R R R 3 3 0x03}

As stated in the beginning of the kmap.txt file (on my device anyway), the third character in the above entry is "VK2 -> key to interpret when for press and hold". So simply change this to:

{R R 3 3 3 0x03}

Reboot your device and try it in messaging - when you hold the R key you will get a 3. Now you need to do this for every key! I haven't done it for the whole keymap yet - this might be the sort of thing you want to do personally, it's not THAT big a job I guess.

Cheers.
1. HTC Snap with stock CE OS 5.2.21048 (Build 21048.1.6.3).
2. HTC Vox
3. HTC Kaiser / WM6.1 (ROMeOS v4.5.3) Radio 1.71.09.01 / Hard SPL 3.29 (relegated to TomTom duties only!)