I see no purpose for it. Losing the average mare for breeding is extremely rare - issues during birth almost exclusively target the foal in comparison to the percentage of mare loss. However, breeding a 21 year old maiden mare drastically increases that risk.
In my honest opinion, it's a waste of time. If the mare was that good, she'd have been bred by now. There's nothing you're going to get from this breeding that you can't probably purchase for less due to issues with her fertility and additional vet exams to ensure her continued health. You are taking a huge chance that will still cost you several hundred dollars when it is very possible she is already infertile or have great difficulty conceiving. If you do get her in foal, the budget will increase to several thousand, and for the risk you're running on both her life and the foals life?
If you're deadset, ensure you discuss it with a vet, but I can't think any vet would condone breeding an aged maiden mare. Do not treat her like a young mare - yes, we ALWAYS risk loss but a 5% risk differs hugely from a 50% risk.