First, being in a cloud without an attitude indicator is a bad idea, period.
Second, you don't have to be in a cloud to be slammed around by it's turbulence, especially near a front...so flying the gaps isn't a good idea.
Third, it's a known fact that convective activity in the higher latitudes can be just as severe as the lower ones, even if the cloud tops aren't as high (translation - a TS in Europe at FL250 can be just as dangerous as a TS in Mexico at FL600). So you don't have to have a super tall puffy to be in danger.
Fourth, assuming most WWI fighters didn't weigh much more than a rigid frame ultralight, I'd stay away from any cumulus...stratus might be okay, unless I saw a 'sea crest' like effect in them, that's usually a bad sign.
FC