Surely it's only a case of language and emphasis that needs to be dealt with here? Make it so that a Ruler 'attaches' themselves to an army. When they do, have an army-wide auto message saying what a great honour it is that the ruler wishes to campaign alongside the Holy Host of Keplerstan or what have you. Bring a little pomp into it and make it mean something. Really emphasise the fact that the Ruler occupies another planet compared to your average noble. All of a sudden instead of being one among many, everyone wants the prestige of having the Ruler campaigning alongside their army.
