I've given it some thought but I can't come up with an easy alternative - sorry.
I don't agree with WW's assertion that these things merely make noise - he perhaps has bought only cheapo ones. These things not only extract smells outside but more importantly extract water vapour from the hob straight outside, at its source - an important point as evidenced by the number of folks whining on about condensation on their walls in this cold weather. Failure of extract from open, boiling pans is exactly why this happens.
I admire OG's creative solution but would have the same concerns as you - if your conservatory roof is glass DGUs it's a no-no, and if you had a hole cut around in the plastic polycarbonate roofing types, I'd be concerned about creating water leaks around the join - polycarbonate expands and contracts a lot with temperature variations. Besides, I think it would look naff.
Unless you have another external wall to the kitchen and could route the extractor pipework BEHIND the run of units (or above them - so you can't see it) to the area where a new drilling to the external world could be made?
I'm referring to use of this sort of stuff -
http://nextday.diy.co...e.jsp?productId=14118
- which although it says is 100mm x 98mm in cross-sectionm I reckon is a misprint - you can get it 100mm x 50mm. That should surely go behind the units.