I've contemplated using TMWWBK for the Taiping, for which jingals would naturally feature. My inclination is to have full-sized jingal units, which act more or less as Irregular Infantry with Obsolete Rifles and which are Poor Shots (this being in a context where most non-European troops will be using Antiquated Muskets anyway); I could see the case for going up to Modern Rifle range and in fact I may well alter that eventually. I personally wouldn't have them reduce cover however.
In fact, to further specify, I'm planning on having units not be single-weapon affairs but instead represent company or even battalion sized units with mixed equipment. In the Taiping army in particular, jingals and polearms were used together, so I would have a unit that has Fighting 5+, but Firing 6+, with the Obsolete Rifles range.