Again sorry to bump an old thread but this comes up when you Google Diconal. Anyway both cyclizine and methadone are orally active so it should work. The best effects are from the general synergy, combined effects of two relaxing depressant drugs. I belive also the relieving of the nausea/itching from antihistamine effects make opioid high more "pure" in terms of much less side effects, and so the combination just makes the whole experience stronger and more perfect.
Also antihistamines can moderately increase the time it takes your liver to get rid of certain drugs (especially those metabolism via CYP3A4), so could further potentiate duration and peak concentration.
A similar effect can be had from combing a strong opioid and a sedating antihistamine (cyclizine is otc in UK, and so is similar diphenhydramine)