My best guess would be probably not, since a lot of small transformers seem to have warnings about installing them in areas like that.
The only mention about Ambiance transformers in high ambient temps I can find on the Seagull website is in regard to their 24-volt dual output model 9465-12 transformer, which the specs say... "24v 1000w model has a high ambient rating for use in areas up to 55*C (132*F)".
The thing is though, that even around here, in the summer months an attic can easily get well over 132*F, so that could lead to issues.
I would maybe shoot an email to Seagull and see what they have to say.
BTW, check out the price on that 9465-12 transformer... MSRP: $585.75