A function like this should be defined for your plugin:

   function! s:rpcrequest(...) abort
     let l:job_id = jobstart(['/path/to/exe'], { 'rpc': 1 })
     return call('rpcrequest', [l:job_id] + a:000)
   endfunction

Then you can invoke s:rpcrequest the same way you would call rpcrequest on a synchronous callback of a persistent plugin.