I found the fixed do idea rather intriguing, where you just sing the solfege note name with vocal adjustment for sharps or flats. It certainly fits well with the absolute pitch aspect (I can use a starting piano note to ensure I'm in the right place). However, I found it confusing, as I have assimilated a lot of the moveable do relationships, such as mi-so being a minor 3rd etc, and it's tricky when it suddenly becomes a major third in the key of Eb, etc. So I've been using another approach, just singing the note names, as they appear on the staff (i.e. A, B, C, etc) but adjusting the pitch for sharps and flats – which is effectively the same as fixed do. I expect that I'll mainly keep using moveable do, but the other way is certainly interesting to experiment with.