Enabling a spit valve is a great idea, David. Reliable authorities have reported that the alto Mylodica actually contains the Melodyhorn. I have the soprano Mylodica, which I know contains the Suzuki S-32C soprano melodion (I have another S-32C). The S-32C inside my Mylodica has been sealed off at the end inside the wooden case (by melting it, I guess), so that major surgery would be required to tune and also (on both the inner melodica and the wooden casing) to create a new spit valve mechanism.
I thought about options when I first opened up my Mylodica, including the possibility of replacing the inner soprano S-32C with a Suzuki alto M-32C and somehow opening up the end of the Mylodica so the M-32C remains intact with its spit valve accessible. The M-32C and the S-32C have the same dimensions. I really like the M-32C but don’t care for the S-32C. So I think we’ve thought along the same lines. I’m interested in any thoughts you might have about how to modify the wooden case. I have a friend who is a wood carver, and I’ve thought about asking him.
Another possibility with your Mylodica might be to find a good 32 (or 34?) key melodica with the right dimensions and a workable spit valve for this application, one that would fit and leave its spit valve exposed in the extra keyboard space, with a vent hole opening at the end or in the bottom. The general idea being to mount a smaller melodica in the Mylodica box with some working space for its spit mechanism. I’m probably not crafty enough for such a project, though.