Keep declining. You say it's not about the gifts, and to a point, it isn't - a shower is also about getting a chance to see the MTB, congratulate her, etc. But if you're not even there.... well, then that even more defeats the purpose of a shower. And Skypeing in is simply not the same.
And also to what you said - if people want to send gifts, they will. They don't need to be "told" to by getting an invitation to a shower that you can't even be there for.
I think the "feelings" of the majority - your friends and family - needs to matter here more than the feelings of one person, the offered hostess. Meaning - there WILL be people a little put off by the fact they are being invited to a shower that you won't even be at. Some may not care, but some will. And "some" is more than "one" and that's what you need to think of- how will this come across to the majority, not just your hostess.
I would offer, though, that when you do eventually come home for a visit, you'd love to work w/ the hostess on throwing a "meet the baby" party where everyone can come and do exactly that- meet your child.