[Anthon]

1. Expressions;
a) apply Time Remapping to the nested comp
b) select the Time Remap keyframes, and apply expression: Property > loopOutDuration.

2. render out a proxy of the nested comp, and loop this. will loop faster and more efficient for preview, but still has to be overridden when rendering out final version.

if anyone has another way, please let know! weird limitation, for which I cannot see a reason.