Part of the problem is there are no figures for Hardness and Hp of armor. There should be. Wooden plate mail would be a bit lighter (in weight, not armor category) and less of a pain to swim in.. but it would be -very- difficult to make, compared to metal armor, it would be alot easier to damage, and without the assistance of magic - you just flat out would not be able to repair the various nicks, chips, dings, etc that the armor would get. If a plate gets split, you can't really just take it off the suit and reforge it, you'd have to get a new plate.
Mending, Make Whole and Wood Shape would be handy things to keep around.
But, as to the questions themselves~
So...what kind of stats would wooden platemail have prior to being the subject of an ironwood spell?
As platemail, with -2 Ac. Guideline for that one coming from the Transmute Metal to Wood spell. So, Wooden Full Plate would be Type: Heavy, Armor Bonus: +6, Max Dex: +1, Armor Check Penalty: -6, Arcane Spell Failure: 35%, Speed: 20'/15' (x3 move while running).
Given the specialized nature of the armor, I'd probably also make it more expensive to make, possibly to the tune of double price. Wood is cheaper in bulk than metal, but you can't reuse any peices you mess up on. That, and finding an Armor Carpenter would kinda hard, normally.
If it were masterwork, then yes.
Does it require an armor proficiency on the wearer's part (namely Druid)?
It would require the same Armor Proficiency as the type of armor it was styled after. So, Wooden Full Plate would require Heavy Armor Proficiency, and a Wooden Breastplate would require Medium Armor Proficiency. Normally a druid has Light and Medium Armor Prof.
The plate would be a little lighter (between 16% (diffrence in small shield weight) and 33% (diffrence in large shield weight) lighter), and you'd still need Heavy Armor Proficiency in order to wear in without major problems.
