Wind is tough, windshear layers work but are costly. What size outer pants do you wear? Are they big enough to fit another layer without being tight? Tight is not good. I think I have a pair of windshear fleece pant liners here that I don't wear, if they fit I'll send them to you. They are bulkier than merino long underwear, so you have to take that into consideration if they will fit under your wool.
Another option is to just put another layer or two of merino on over the first. If your military outer wool has a loose weave, the wind will still get through though.