I think brushing clear coat onto weathering powders will do precisely what you think!
If you aren't likely to handle the model much after weathering, you may decide not to varnish. However, a varnish coat would protect the model and weathering. I am loathe to point you towards aerosol can varnishes, as I have indifferent experiences with them, but a good brand other than Humbrol (sorry!) would do the job an airbrush would be used for, but with less finesse.
As an alternative, have you considered dry brushing? I'm sure there are plenty of tutorials on the subject of weathering and dry brushing on the internet if you search around. Dry brushing involves removing as much paint from the brush as possible, so that when you gently stroke the brush on the model hardly any paint is transferred. With care, it's possible to build up layers of paint, such as exhaust staining and dust or dirt deposits, to the level you think is right. It's a skill that needs practice, and which in my experience needs a paint like enamel that doesn't dry too quickly. It may well be worth experimenting on something other than the Thunderbolt until you feel confident.
Perhaps someone else more familiar with drybrushing acrylics might like to chime in?