As a landless ruler, it does not look like it. It may be intentional though, as the point of this system is to not have the Monarch have anyone above them, and them being part of an army would mean that someone is "above" them.
On the flipside, it also means I can't un-army myself.
Yeah no. If we go to that extreme we run into issues with mixed power structures. For example should the General be able to be in an army unless he is the Marshal of that army? Should Marshal need to be of a higher rank then anyone within the army, thus "Knight" Marshal can only command a army made up of knights so has not to embarrass Lords and Dukes by placing them under the authority of a mere knight? It may make logical sense (or may not) but I seriously doubt that restrictions on this level are going to be fun.
