Zip ties are awesome and all but I wouldn't recommend them for that application. Its constant high stress/fatigue situation, they will last a while but I wouldn't consider them an appropriate replacement. Hog rings are perfect for this (that's why upholsterers use them) and even if they are stressed they just bend rather than failing. And when the zip ties do fail its going to make your covers loose, which is going to cause your foam and all the velcro attached to it to fail.
Why do it halfway? its an $11 tool and 15 cents worth of hog rings.
The heating element will probably fail faster than the zip ties, I'm already on my second one, at this point im just wondering when this one is gonna fail.