hide wp installation behind running html site

hi all,
some of my clients have an HTML Website and then they want me to make a wp site for them.
using a wp an a subdomain is a headache, I have tried this and since wp saves relative paths, one has to do a lot of work making it all run well.
using the maintenance plugin or something similar is just giving me a landing page, but how can I work on the same domain and hide the wp install while the html site still visible?
any ideas?
thanks!!!
nili