Has anyone set up HSA plans to allow employees to adjust their per-pay period amounts themselves?

Hello all - we currently have employees enter their annual contribution for HSA during Open Enrollment. We have had a request to see if they can enter per-pay period amounts and the ability to change them throughout the year without having to call HR to adjust it for them. We have the min/max limits set up in configuration so that they cannot enter a contribution greater than the max.

Here are the scenarios presented:

Example 1:

I would like to contribute $500/period from January to May/June when I have increased medical bills due to the need to meet my high deductible.  Then mid-year when my deductible is met and my medical bills are lower, I would like to contribute a new amount (that I would calculate on my own) such that I have contributed the maximum to my HSA for the year by November.  This way I have extra spending money for the holidays.