Given that the pom has a correctly configured scm tag section then it's just a case of finding the browsable file.
Normally mercurial repo's are exposed using hgweb.cgi or hgwebdir.cgi.
Not sure about hgweb - use hgwebdir myself - but - the trick is to grab the raw file version of the tip revision. Once continuum can grab this then it will use the scm tags to get the project.
https://server/hgwebdircontext/project/raw-file/tip/pom.xml
NOTE
When you run hg clone to grab the repository it will always check out the entire repository.
Mercurial - like some other distributed version control systems works such that each developer has a full value repository - there is no real master other than by convention in a team.
To set up your maven project so that the scm configuration points to a developers local repo:
<scm>
<connection>scm:hg:file://${basedir}</connection>
<developerConnection>scm:hg:file://${basedir}</developerConnection>
</scm>