Check number cannot be stored for custom payment instruments

The field "check number" is not stored for any user-defined payment instrument, even after customizing tpl file to show the check number field for all payment instruments. (there is some javascript that hides that field from the user if payment instrument is anything but the core-provided instrument called "check" )

To reproduce:
1) Create a new Financial Account of type "Asset". Name it "Chase Bank Account for restricted funds"

2) Create a new payment instrument. Name it "Check for restricted funds" and associate with the Asset account from the previous step

3) Click "Create New ... Contribution" and fill in the usual contribution details. Choose payment instrument as "Check for restricted funds" and fill in check number. (this assumes tpl has been changed to show this field for any payment instrument)

The contribution is created, but the "check number" data is NOT in the database record.

(FYI: Curious how other organizations deal with accepting checks that get deposited to more than one bank account)