My father's got a jet wash that has a bottle attached that you can put the shampoo in. The manufacturer recommended their stuff but that's to be expected, not sure if you can put any stuff in though, if it's too thick then it may not mix properly, if it's too soapy you could just end up with loads of foam coming out.
The jet wash would be useful in this weather to get the crap out of the wheel arches though and should make rinsing the soap off a bit easier, plus you can use it to clean the drive and slabs around the house so it would be useful for other stuff