Coating to make waffle cones withstand hot liquid?
Many hydrocolloids are very heat tolerant.

Hydrocolloids Primer

Some of the options that might work for you are:


Agar agar
A polysaccharide made from boiling seaweed, agar agar is heat stable up to about 90C. It gels when cooled to 40C. It is inexpensive and easy to find, especially in Asian markets. It would be relatively easy to coat the inside of a cone by heating the agar agar to 90C, cooling it almost to 40C and pouring into very cold cones and pouring out the excess ungelled liquid.

Obviously it won't work if your liquid is hotter than 90C.
LM Pectin
Like agar agar, pectin is a polysaccharide but it is extracted from fruit. Regular, off the shelf, pectin that requires a lot of sugar is HM Pectin. It will melt easily. LM Pectin on the other hand, requires calcium to gel. Once gelled it is heat stable and will not melt again.

Unless you have a well stocked specialty pastry supply store handy you may have to buy this online and it will definitely cost more than the agar agar.
Sodium Alginate
Like LM Pectin, sodium alginate gels in the presence of calcium. Often used to make "caviar" preparations of random liquids. It is easier to find for sale as part of molecular gastronomy kits.

It forms a heat stable gel but you'd have to be creative about how to coat a waffle cone with it. Perhaps you could form a sheet of it and "glue" it into the waffle cone.


All in all, if your liquid is cooler than 90C, for cost and ease of use I would recommend going with the agar agar.