Great article, but I'd add one very important synergy for Stonemason: cost reducers.
We all know that cost reducers work well in games with +buy, and it's no different for the Stonemason's virtual +buy. Using a cost reducer (Bridge, Highway, Quarry) before buying Stonemason can get you a lot more mileage out of the overpay - your cost reducer is basically working triple-duty. Not only do you get the SM for dirt cheap, you can hit your overpay target much easier. Couple this with regular +buy, and you have a nice little combo that can score lots of expensive cards on the cheap. The only thing to look out for with cost reducers (as you mention with Peddler) is that you don't drive your overpay target down to $0, which would render it un-gainable from Stonemason.
Here's a nifty play where I pick up 4 Nobles and 2 Cartographers with my 3 Stonemasons, and it's only Turn 5!
BadAMutha - draws Squire, Squire, Quarry, Quarry, Copper
---------- BadAMutha: turn 5 ----------
BadAMutha - plays Squire
BadAMutha - takes 2 actions
BadAMutha - plays Squire
BadAMutha - takes 2 buys
BadAMutha - plays Quarry
BadAMutha - plays Quarry
BadAMutha - plays Copper
BadAMutha - buys Stonemason
BadAMutha - overpays for Stonemason with 2 coins
BadAMutha - gains Nobles
BadAMutha - gains Nobles
BadAMutha - gains Stonemason
BadAMutha - buys Stonemason
BadAMutha - overpays for Stonemason with 2 coins
BadAMutha - gains Nobles
BadAMutha - gains Nobles
BadAMutha - gains Stonemason
BadAMutha - buys Stonemason
BadAMutha - overpays for Stonemason with 1 coin
BadAMutha - gains Cartographer
BadAMutha - gains Cartographer
BadAMutha - gains Stonemason
http://dominionlogs.goko.com//20131015/log.513bdc80e4b0da6e3de056bb.1381862770589.txt