I like it.  Not for serious holdings, but for what you seem to be doing.  I did this in Robinhood back in the day...

If it turns out they are fractional?  Then I would frown on it.  But if they are actually custodying BTC then yeah... doing something small to help make them successful is not a bad idea.

And if they chose to have a fractional BTC reserve?  It would very possibly be their undoing.