[Peter Goulding]

I would tend to side with Roger here. In sestinas, such creative puns are encouraged, provided they are a recognised syllable. So whereas constrains would be no good for rains I reckon abstains is ok for stains
(Okay, it's not a sestina, but you get the picture)

Though as Jayne says, the past history of the judges would be more informative as to whether words qualify, rather than my opinion.