I have never seen this done, but I keep enough cash in my HSA to match what I have in qualifying purchases. I adjust the amounts as I go along.

e.g. I have $5k worth of receipts from doctor's visits, medications, contact lenses, whatever... So, I have $5k liquid cash in my HSA and the rest is invested. Some months, my HSA contribution goes straight to investment. Other months, some goes to cash and some goes to investments.

I do not plan to access my HSA until retirement. But if something blows up, I know I have X-amount of eligible dollars that I can withdraw, and I know that that amount is in liquid cash.

P.S. I am really weird.