Interesting dilemma.
In my case, my wife enjoys my playing. She has no clue of what my guitars are or what they are worth, but she knows how to count. Seriously she didn't even realize the difference between my sunburst Guild and my blonde Taylor, or what they are worth - compared to my Epiphone. This is in spite of me being up front with her and telling her what I paid for my Taylor AND my Epiphone when I bought them (within a year of each other). I already had my Guild. All she knows is that, including my GS Mini, I have four acoustic guitars and one electric.
I have already planted the seed that (after I find a new job) I will be searching for a my next guitar, likely a 12-string or possibly a 000 size with different tone-woods (cedar? mahogany?).
Since you already tried the tonewood conversation and the didn't go anywhere, perhaps you can use the alternate tunings logic. Make sure she understands the terrible wear and tear that constantly changing the tuning from standard to alternated has on the bridge, nut, and tension on the instrument and how costly it can be to fix in the future :-).
Maybe that will work.
I always believe that honest is the best policy. In my case, my wife either has selective memory of my guitar endeavors, or (more likely) she is just not that interested. She only likes hearing them, and probably can't really distinguish the differences in the tonal qualities. This is probably a good thing in my situation.
Good luck,
Jim