Splice a 45W MagSafe 2 connector onto an 85W PSU?
The price difference between Apple's 85W and 45W power adapters has always been very small, which one could assume means that they probably use the same high quality components (wiring and connector) for both, with the only difference being the size and output of the adapter. This also makes sense from a manufacturing perspective, but naturally we will never know for sure unless we get a statement from Apple about it (which, of course, we can assume we won't).

Considering the above, I certainly think what you're asking might be possible with the right amount of skill, but the question you need to ask yourself is if it's worth saving a few bucks for a very likely fire hazard in your home (or wherever you would use the adapter).