is it possible to model perennial plants at all? (plants that live for more than 2 years)
Nope.
I assume one could use the proper tags to plant something in a farm and instead of harvest it, it would drop its products?
Nope.
unless there is a way to harvest a plants products without pulling it out of the ground.
Nope.
I am trying to mod plants to have real life growth times...and that works just fine for plants that you plant and harvest every year...but there are a lot of plants that you simply plant and harvest the product every year (or multiple times a year) without replanting
Word.
Toady hasn't done a whole lot towards making plants work realistically - he seems to care mainly that they exist.
I've got a WIP which modifies growth seasons, though the seasons have to be within a single year. For wild bi+annuals, I try to just think of each plant as being a "stand" of plants - some coming to maturity each year. For farmed plants, there's really not much you can do - especially with plants that should be overwintering, as the game will (as of 0.40.24, I haven't checked yet in the new versions) literally
not let you plant seeds if they wouldn't mature before the end of the year.
You might want to take a look at my plant mod, Expanded Plants (nee Plant Fixes), and/or its
table of plant growth times on the Dwarf Fortress wiki. I've got all the data collected, a scheme of additional reactions, products, etc., and have modified raws for ~1/8 of the plants so far, but it's a bit of a slog, and I feel like I need to get the Modest Mod updated for the new version before I can get back to work on Expanded Plants, since a lot more people use it.