If you are drenching right at the trunk, it is very unlikely to be taken up by other plants unless they are right at the base of the tree...imidacloprid doesn't move much in the soil.
"Honey bees are dying off in huge numbers because of imidicloprid" So the theory goes...only problem is there is no evidence that this is actually happening. CCD was blamed on imidacloprid by individuals who have already made it clear they don't like any pesticide. Real researchers were not able to find any correlation.
Having said all of that, if you and/or your clients feel it is most prudent to inject into the tree, that is fine, and I think it will work well. Nothing against Maguet, but since I already have a TreeIV, I'd probably be inclined to use that if I did want to inject imidacloprid. Mauget would certainly be easier clean-up though, so I'd have to look closer at price differences it if I went that way. My biggest concern would be annual drilling. I am hesitant enough to drill every other year for Tree-Age, but a drilled tree is better than a dead tree!