Piotr wrote: 
I've been looking forward to your contribution hoping you would tell us someting about these diphthongs. I'd be interested in two things: from the descriptions I've read and from my own informal observations of spoken Lithuanian I gather that they are in fact long mid-high vowels ([e:] and [o:]) preceded by semivocalic onglides. If so, something like [je:], [wo:] could do as a rough transcription. Do you agree?

The most interesting peculiarity of Lithuanian here is that the term semivocalic may not suit (at least in the meaning applied to English [j] and [w]). They are so weak and momentary, that I would rather called them the 'way [e:] and [o:] are articulated in the first stage of articulation' (somebody, please remind me the term!).
 
Sergei