Agreed - converting to disc in front will cost you a lot more than simply buying that Orbit fork and swapping it right in.

I guess you could check with Orbit to see if they sell the same fork but in Disc option to see what the cost difference is. Or perhaps they have one with both Disc and Brake posts on it? That way you could upgrade to disc in future if needed.
Or bring a higher resale value should you choose to sell the bike in future...

Others on this forum might express concern about adding disc brakes to a bike that was never built with a disc fork in mind. Since adding a 180 or 203 rotor at the end of a long lever can put a lot of force on the frame.
Especially if the frame designer wasn't expecting this. I bet if you ask Trek, their answer would be no.

However I can see from pictures the T900 has an extremely long head tube to accommodate a range of rider heights and appears to be nicely welded to the frame tubes. Hence it appears the risk of adding a disc in front might be a bit lower than other tandems I've seen people do this to. Maybe others who have done this mod already can chime in.