Feasible? That word can have different meanings to different people, for me a jetjon was feasible because I fish shallow rocky clear rivers and thats pretty much all I have other than the Ohio river. If I was a lake fisherman then a jetjon would not be feasible solution to a good fishing boat. You mention stumpy tree filled waters, in that scenario a jetjon would be fine since there isnt a lower unit hanging below the boat waiting to be knocked off. If your water is muddy and debris filled then an outboard would be a better alternative than a jet pump (think of a pump as a water vacuum).
As far as build design goes, I agree that a weld-in aluminum pump is superior to a cut and splice fiberglass install, however....not everyone has the tools or know how to weld or for that matter the availability of an aluminum pump housing (only specific models have this aluminum design). For a first time build for grins and giggles then I would suggest a splice and glue method detailed on this site in many posts. This way you can utilize the pump, engine mount and exhaust setup in the existing ski without having to fabricate a new design, and the cool part is, IT WORKS GREAT. Time will only tell how long these builds will last but suffice it to say they work well to get you out on the water for cheap. My next build will be a more permanent weld in on a nicer hull, but for the little money I have in my jetjon, I couldnt go out and buy a decent outboard fishing boat. So to summarize, yes a jetjon is feasible and very affordable if you can buy the parts right, however the real cost comes in time sacrificed getting the thing built correctly...they are fun but very labor intensive.
Good luck, you came to the right place for info there are a quite a few guys on here that definatley know their stuff.