Yes. MY OTC has several Officer Cadets who have passed a PTI course, and act as our PTI's. People who want to do it put their names forward and went to do the course. Think a couple of them also work with companies like ODFC and BMF as well, so it's certainly possible.
Yeah it's a TA course you do but when I did it there was quite a few officer cadets on it. The first module is 2 weeks long and when you finish you get cross swords and can take PFT, CFT, MST, Steady state run, obs course, tabbing and circuits.
Oh one other thing. You can do an ETL (Endurance Training Leader) course in the OTC through your brigade. It is a 2-day course that sets you up to take steady-state runs back in your unit. It's like a mini-PTI course and lets you help the PTI (organising markers on a tab or SS run, risk assessments etc.) however probably an essential stepping-stone to advancing towards your PTI course. Speak to your PSI as they are frequently ran.