I assume from your wording that you are looking to do a fairly "straight" transcription, not a paraphrase - doing a typical Liszt showy-type paraphrase requires different techniques. A few thoughts..
I would suggest looking through transcriptions of two well-known arias: the Liszt transcription of Isolde's Liebestod and the Thalberg transcription on Casta Diva (from L'art du chant, op. 70, it's no. 19 in the set) and seeing how they lay it out. To me, the most obvious aspects are Liszt's use of quiet tremolandi for textural reasons, and the way in which the two hands are used as one unit within which the melody may move (i.e. sharing the melodic line between the hands: this is a Thalberg trademark).
I suspect the effectiveness of the end result will depend on how well you lay out the arrangement and on how well you can bring out melodic lines within the hand (remember that often one hand may be playing both melody and part of the accompaniment texture). I'd suggest that, all other things being equal, the denser the orchestral writing, the harder it is to arrange effectively.