Bleh! The thought of the taste in my mouth for that long would be the end of it for me!
On the metabolism and half life though, no idea. Most medications that have high "first pass" metabolism by the liver are designed as sublingual or not oral to avoid it. I have heard of people having half dose spiro in the morning and half a night (ie. instead of 100mg in the morning only, 50mg in the morning and 50mg at night), which would technically give you a more continuous level in your blood rather than a peak and trough from a daily dose, but I'm not a doctor.
Speak with your GP or Endo but as with most things medication wise I don't think there is enough research out there to say one way or another.
Again, spiro sublingually or buccally just sounds awful!