Like most aeroplanes, a flexwing could theoretically fly a loop - by diving as near to Vne as possible, pushing the bar out hard, whilst maintaining full throttle. You'd have to do the flight mechanics maths to double check that for a specific aeroplane, but I'm sure some could - indeed I've seen video of it. Similarly, I've seen film of hang-gliders looping.
Climbing vertically is a different issue - it requires a thrust:weight greater than 1 - I don't think that any microlight (or indeed the majority of fighters when at a warload) has that capability so any vertical climbing would be very short-term until you ran out of energy (a few seconds at most for the majority of microlights).
For some very good reasons why you
shouldn't attempt to loop a flexwing, search on the terms "flexwing tumble"; if you really want to get into it, read
this (5Mb pdf).
G