I think if you're going to use a conlang for a relay that is a) very limited, b) not well described, or c) stuck in a social/historical context that gives it no chance of translating the text, then you should say that up front. Imagine encountering Toki Pona for the first time in a relay and having no idea what it is. You'd be forgiven for assuming things about the torch that aren't necessarily true.
As for the length of grammar, I think a full reference is fine, but maybe give a brief primer, or bold the parts that will be most useful.
2024 Translation Relay
Re: 2024 Translation Relay
Way back in the beginning days of me conlanging, when this forum was on zompist rather than verduria, I took part in my very first translation relay...and when I submitted the relay to the person in line after me, I included everything I had come up with for that conlang of mine...all the entirety of the lexicon, the grammar and what little syntax I'd made for it, etc...I don't think it reached 5 pages, but it was - certainly in hindsight - more than I should have submitted.
afterwards, I was sent a message to the tune of "thank you for your enthusiasm, but please don't do that again" (not an exact quote by any stretch)