Now I'm just pulling this out of left field, the dimensions of the sight, the rail, and the back up sights might not allow it but here goes......
What about HK MP7 sights? They arent too terribly big, and when folded down, they have backup "pistol sights" that stick up. I wonder if these rudimentary 'pistol sights' would stick up far enough to be visible through the T1's tube? Folded upward, they would be too high, but folded down, the backups might just be tall enough. Not accurate for anything but backup, but then again thats all youre looking for, right?
Just a thought, again I dont have a PS90 on hand so I cannot try this
Found this pic on google, may be real may be a toy, but either way it appears dimensionally correct. This should give you an idea of the height-over-rail of the MP7 sights