And it's not even a tax write-off until you have enough income to offset expenses. (Which is nice as far as it goes but is small scale where you can write-off a few $K in expenses against a few $K in income.)
If there's already income paying the pesky mortgage, you start up an official business as a side hustle. As long as you are showing income even if at a loss, you then get to use that loss as a deduction. If it never pans out to be profitable to the point the tax man strongly suggests the business should close, you close it. In the mean time, you've followed a passion, that even as a loss, still gives financial benefit helping with the pesky mortgage.
There's doubtless some wiggle room but it's like home office deductions. Triggering an audit by the "tax man" is rarely a good idea for most people and may well cause accounting bills that exceed any marginal gains.