Welcome to MLS. There is a great bunch of people here with a wide range of interests. I find that the folks here are SOO much more tollerant of differing opinions and tastes than anyother site I've come across in Model Railroading. There are gardeners, rivet counters, whimsical fans, live steamers--all living in harmony. I mysefl am sort of in your boat--a model railroader that choses to run outside: 1 because I like the fresh air and 2 the size of the trains is addictive--I like my trains with some heft.
That said, what you propose is not all that unusual. Here is a link to a thread on my layout I am building, and other than the fact that it is in the dirt--it is a model railroad outside, and I might plant a few live plants if I have room (at this rate doubtfull).
Sorry still don't know how to make links active--just cut and paste into your browser.
The man's work you really need to see is Richard Smith. Do a search for his Port Oregon Coast railroad--there is even a PDF file put together with its construction. It is raised on pressure treated benchwork and I think it MAY have a few live plants, but Richard himself admits to being like you and me--not a gardener. Sorry Richard if I got that wrong, but I recall seeing such a comment in one of your posts--it made me feel at home. If it is green I can kill it. Not sure how my grass stays green in the yard--but then again St Augustine grass is really a weed--I believe you folks up north call it crab grass. I call it easy to care for!
Good luck with your plans, I can't wait to see what you come up with.