By law, all microlights are prohibited from pitch angles greater than 30 degrees, which limits all but the most basic 'unusual attitudes'
Not quite true - CAP403, the display requirements limit microlights being displayed to ±30°, and quite a few types (mainly I think Mainair flexwings) also publish this as a hard limit - but it's not a legal limit.
Mind you, there is a legal 60° bank limit (although a few types have a stricter limit of 45°) - arguably at top of loop you have 180° of bank!
Any loop in a microlight, 3 axis or flex wing, would not be a true circle, as much due to the fuel starvation issue as the aerodynamic / mechanical design limitations inherent in the flex wing design.
Most loops should stay in positive g throughout, so there shouldn't be a fuel starvation issue.
G