It is possible. You have to use some troops IMO. Companions will be no good, except of those trained to level 20+ and with superb equipment. Make them all Knight/Mameluke like, add some other HCav (or cheap Khergit units) to lower the ratio in battles (it would be a trouble trying to kick King's ass, when among 100 people on the battlefield 6 are Yours...) and You'll be able to do it. Of course You should leave Yourself one "friendly" faction, where You could sell the loot and rest. Khergits would do the trick, using only Lancers should be easy to maintain (they are quickly trained) and in "small" groups (less than 50) should be able to give You advantage of picking Your fights. You'd probably avoid sieges, so lack of infantry is not a problem. Picking the fights should be the key here. It's a pity that, in Native, You couldn't join in the battle between lords if You're not merc or vassal, this would give banditry nice advantage, but well, all the idea could be fun.