Do you need different credentials too (username/ticket), or just a different workspace?

For the moment (1.6.1) you could use p4groovy.  The p4 object requires a workspace and credential, for example (not tested):

stage 'Unshelve some files' {
    ws = [$class: 'StreamWorkspaceImpl', 
        charset: 'none', format: wsval, pinHost: false, 
        streamName: streamval]

    // Create P4 object
    p4 = p4(credential: credval, workspace: ws)
    p4.run('unshelve','-f','-s',"${REVIEW}",'-c','default')
}

Yeah, I would need to specify what credentials i want to use as well (since i may sync/unshelve changes from multiple servers). 

I will add this our backlog and give it a go when I have a spare moment. 

Thank for the update

I believe I'm being hit by the same issue in 1.6.1.

Our workflow script uses

checkout(scm: [$class: 'PerforceScm', ... workspace: [$class: 'ManualWorkspaceImpl', charset: 'none', name: "build_${env.JOB_NAME}_${env.NODE_NAME}", pinHost: true, ...

to check out the tree on a whole bunch of nodes (as we need to suppress changelog generation and polling). When we call immediately after

 p4unshelve resolve: 'theirs', shelf: params.SHELVED_P4_CHANGELIST_NUMBER

{{}}, it always uses the workspace of the first checkout that occurred (before the parallel stage). This has only shown up as an issue when there's files being added or deleted in the shelf, as it then refuses to deal with them, and in fact appears to lose the executable flag on them, if present. Otherwise, 'resolve their's appeared to override the overlapping changelist issue.

Assuming that my problem is correctly part of this issue, I'll try the workaround show above (now that I know what the 'p4' step is for) and see if I can get it to do the right thing there.

Mark as an improvement.

Add workspace and possibly credentials to unshelve option.
In addition include unshelved changes in build summary.

Note for anyone trying the workaround described in the first comment:
"${REVIEW}" must be "${REVIEW}".toString(), or script-security-plugin will fail trying to find a method to match, with the same stack-trace as JENKINS-37527.

The failure is that Array.set cannot coerce a GString into a String.

I raised JENKINS-44557 against script-security-plugin for the exception requiring toString() to be used.