To the best of my knowledge something has to make glycol, water or actual steam to be effective. Maybe improvising with a wall mount Paloma-Style could accomplish similar to a boiler??? Many brands besides Paloma but similar compact mounting on a wall etc.!
I found something interesting on Amazon for tiny areas. Note that after scrolling down to
product description when you get to the below link ... as to how many BTUs are transferred per water temperature. So it is not like it has to be even hotter than 120° F to do some good.
Based on the water temp - BTU chart ... I am thinking that some of us could even improvise with a compact wall mount unit like 2Biz & some others are using. Of the below example would not be big enough fo a tunnel. On the other hand it possibly could be a good fit for equipment room heating &/or for heating a dog wash. IMHO it makes a lot more sense than an out of sight electric bill.
Beacon Morris Hydronic Kickspace Heater, 10360 BtuH Max: Amazon.com: Tools & Home Improvement
www.amazon.com
2Biz & others may be better qualified about the possibility of a Wall Mount accomplishing something similar to a boiler!