while probably not the most ideal solution... i found a quick method to do
this using the builtin converters within the configuration to append the
padding where necessary.
here is an example:
log-format %[var(txn.jwtpayload),b64dec]
http-request set-var(txn.jwtpayload) req.hdr('x-jwtpayload')
http-request set-var(txn.jwtpayload) str(),concat(,txn.jwtpayload,==) if !{
var(txn.jwtpayload),length,mod(4) -m int eq 0 } {
var(txn.jwtpayload),length,mod(2) -m int eq 0 }
http-request set-var(txn.jwtpayload) str(),concat(,txn.jwtpayload,=) if !{
var(txn.jwtpayload),length,mod(4) -m int eq 0 }