Ok, I have figured out the, somewhat obvious, solution.
Using grunt-bump you can update the package.config, you can also update the package.config that is often read into the variable pkg at the beginning of the initConfig. so in the setup of the bump task you specify
{
updateConfigs:['pkg']
}
Then in the xmlpoke I can do
{ xpath:'myxpath', value:'blablabla/<%=pkg.version%>'}
and this works. What I was doing before was
{ xpath:'myxpath', value:'blablabla/' + grunt.options.versionNumber}
where I had set the versionnumber in a previous task after the bump. Or
{ xpath:'myxpath', value:'blablabla/'+ grunt.file.readJSON('package.json').version}
neither of those worked. I guess I was just getting to smart for my own good as the <%= %> is the more common and typical way of accessing parameters from within the initConfig.
Anyway, there you have it. Or I have it.