For this purpose Psion has an utlitity called psionVU . I used it in our XT15 to lock the screen and start up the WTNC
There are other utltities like Wavelink Avalanche Enabler but I prefer PsionVu for this model.
I'd personally recommend the Avalanche Enabler, even if you're not using the Avalanche server component. Beyond being able to control which Application launches at startup you can also restricted access to specific applications and lock down the device. There are also versions available for just about every mobile device platform so you could create a common user experience across different device types if you have more than just Omnii devices. PsionVU is only going to work on the legacy Psion devices from Zebra.