I have a theory that the vigorous shaking involved with making poppy seed tea, combined with the large amount of poppy seed oils, effectively creates a liposome around the alkaloids (ie, morphine) which explains why poppy seed tea is so long-lasting when compared to pod tea or oral opium.
https://en.wikipedia.org/wiki/Liposome
This would make it not suitable for rectal administration as there would be no way for you body to open up these liposomes. Heating may open up these liposomes however which might make it more suitable for rectal use.
Still my general recommendation is: don't plug poppy seed tea.
If you do, let us know what happened (just out of curiosity's sake).