Safely heating an enclosed space

Is there a recommended way to heat an enclosed space?

I had my bathroom redone three years ago. As part of that I very specifically asked them to move the pipes for the tub off of the exterior wall. And a year later when the pipes frozen and I ripped out some dry wall I can clearly see the pipes still run along the outside wall to the tub.

I don't have access to where the pipes freeze. I can just see where the elbow joint is that turns to run down that wall. I can see a little bit down that run too but it is a pretty tight space. I suspect the pipes are freezing about 8-16 inches down that section of pipe. But the pipes go through a stud before then so I am not sure.

The cavity that the pipes are located in is connected to the empty space under the tub. I have an access panel to under the tub on the far corner from where the pipes freeze. (it is a Jacuzzi tub so the access panel is for the electrical).

I can not reach any of the pipes from that access panel.

Under the tub is routinely substantiality colder than the bathroom so heating the bathroom more doesn't really help when it gets really cold. Surprisingly that space is also routinely colder then the unheated garage below it, so my efforts to keep the garage warm while effective for retaining heat in the garage have not helped this problem. If it weren't obviously a bad idea, drilling a hole to let air/heat from the garage into underneather the tube would probably solve this. But let's not go over all of the reasons that is a terrible idea.

The pipes only freeze in extremely cold weather for our region (below 0F-10F degrees) so it only once a year problem. But risking a burst pipe once a year is not something I want to deal with. And I will grant the contractor, that the temperature this is a problem at, is a lot lower than it use to be. My pipes use to freeze multiple times a year.

The section of the pipe I have access to is copper but it transitions to PEX in the wall and under the tub. I think it is freezing after it transitions to PEX.

Is there some low heat heater I can put in the space besides wrapping the pipe in heat tape? Is there a trick for blowing in hot air through a vent to get the warm air of the house to circulate under the tub?

Yes, I know this could have been insulated better. But I am trying to avoid ripping out all of the tiling or climbing up a ladder outside and removing the siding/sheathing. So looking for a safe but effective solution, even if it is inefficient from an heating cost POV.

I could blow in insulation but I dont think it will reach the nook where the pipes are actually freezing so while it would insulate the tub, it wouldn't solve my pipe freezing issue.

The only solution I can think of is using a hot water recirculator but that involves removing tile to get to the pipes near the nozzles. My wife doesn't want me to do that, so trying to think of alternatives before I do it anyways.

But it seems like there has to be some normal way of blowing in warm air into a enclosed space that is appropriate in wet spaces. If I could force air into the space under the tub, I am confident it would also heat the problematic section of pipe. Any ideas that won't burn my house down would be appreciated.