Two years ago I experimented with chia seeds from the shops - the germination was high (I used sterile chitting).
I transplanted a few - I didn't want many - but they were competing with my pepper growing, so they didn't get much attention.
In the end I had 3 in a 10cm pot that did well enough to merit a 20cm pot in my sunroom - they produced beautiful pale blue flowers 70cm tall but no seeds (despite the presence of hoverflies).
Last year I tried putting a similar pot in a sunny spot outside, but they didn't do as well (though this was not a scientifically significant result).
I think your chances of getting a useful quantity of seed from anything less than 3 or 4 square metres of rich soil in a polytunnel are pretty slim. I have better uses for my polytunnel.
If you can mobilise these resources then it's worth a try, but I doubt you'll ever compete with the shops on this crop.
However if I had a few metres of unused sunny bed in the ornamental garden I would go for the double return of edible ornamental by planting out in May.
Cheers.
