I don't think it is the worst of ideas, the longer length doesn't seem to be too much of an issue with the Polarie beyond the capabilities of the tracker itself. Despite what the specs say in the book it can easily cope with 300mm for a minute or two if you ever feel the need to go for that long but as you say at longer FL artifacts like PE will be an issue whatever the lens.
Also looking around there are a number of successful results using the same or a similar set up.
Personally I like the rig in the link of M45.
http://www.singastro.org/forum2/view...hp?f=3&t=10473
http://stargazerslounge.com/topic/13...n-polarie-m42/
http://stargazerslounge.com/topic/13...l-mount/page-2
http://www.flickr.com/photos/53052745@N05/10318590116/
I have seen more ridiculous set ups on the Polarie but I don't have any desire to push things beyond it's limits. I think something like this can sit comfortably within the 7lbs limit.
Other benefits are budget restraints, I think a small scope offers at the least equal budget and certainly in some instances a cheaper option than a quality lens and is almost certainly many miles better than any lens I currently own.
The weight is a plus, they are relatively light weight either comparable or less than some lenses.
It can also double up as a travel scope or spotter.
Like I said it's something I'm considering, I'm not totally sold on the idea yet but it is growing on me.