OK, I had another look and it looks like there's no way to specify flags through the actual XMLRPC interface, hence no way around that restriction. This is weird and may be a bug. So I see 3 possible ways around your problem:
1- Find out how other XMLRPC DW or LJ clients do it.
2- File a github issue about it.
3- Since (I understand) the XMLRPC interface is not really maintained due to the new DW API in development, look into using that for your client.