I have realized that the client type field is not exposed at all in the latest version of the P4Java API, so there's no way for this to be implemented currently.

It is now supported in p4java 2017.1

Planning to add dropdown to Manual Workspace Impl, do you need it on any others?

That's great news Paul! Manual Workspace Impl is the only place we'd need it exposed.

Code changed in jenkins
User: Paul Allen
Path:
src/main/java/org/jenkinsci/plugins/p4/workspace/ManualWorkspaceImpl.java
src/main/java/org/jenkinsci/plugins/p4/workspace/WorkspaceSpec.java
src/main/java/org/jenkinsci/plugins/p4/workspace/WorkspaceSpecType.java
src/main/resources/org/jenkinsci/plugins/p4/workspace/WorkspaceSpec/config.jelly
src/test/java/org/jenkinsci/plugins/p4/DefaultEnvironment.java
src/test/java/org/jenkinsci/plugins/p4/client/WorkspaceTest.java
http://jenkins-ci.org/commit/p4-plugin/91e3a3c8284d2bb84061f1afe2d16f63aeb95ee7
Log:
Manual Workspace support for READONLY and PARTITIONED.

Change Type to Enum.
JENKINS-39753

Ready for release.

Released in 1.7.0

Hey p4paul - I've gotten around to trying the new Type option for ManualWorkspaceImpl and while I can set it to READONLY in the job configuration, it doesn't seem to take affect when it writes out the workspace to perforce. To test this, I deleted the workspace associated with a job (since a workspace type cannot be changed after it's created), used the Jenkins UI to change the manual workspace type to READONLY, and then re-ran the job.

The workspace that gets created with the new job configuration does not have a Type field, and I see no contents in my readonly clients directory on the p4d server. Am I doing something wrong?

Here is a simple Pipeline script using 'readonly':

node() {
    checkout([$class: 'PerforceScm', 
        credential: 'graph1234', 
        populate: [$class: 'AutoCleanImpl', 
            delete: true, 
            modtime: false, 
            pin: '', 
            quiet: true, 
            replace: true], 
        workspace: [$class: 'ManualWorkspaceImpl', 
            charset: 'none', 
            name: 'jenkins-${NODE_NAME}-${JOB_NAME}', 
            pinHost: false, 
            spec: [allwrite: false, 
                backup: false, 
                clobber: false, 
                compress: false, 
                line: 'LOCAL', 
                locked: false, 
                modtime: false, 
                rmdir: false, 
                serverID: '', 
                streamName: '//p4-perl/main', 
                type: 'READONLY', 
                view: '']
            ]
        ])
}

and the verified with:

p4 client -o jenkins-achilles-pipeReadOnly

Client:	jenkins-achilles-pipeReadOnly

Update:	2017/06/08 14:02:58

Access:	2017/06/08 14:02:58

Owner:	p4jtestsuper

Root:	/home/pallen/slave-mac/workspace/pipeReadOnly

Options:	noallwrite clobber nocompress unlocked nomodtime normdir

SubmitOptions:	submitunchanged

LineEnd:	local

Stream:	//p4-perl/main

Type:	readonly

View:
	//p4-perl/main/... //jenkins-achilles-pipeReadOnly/...

I used a 2017.1 P4D and had set the 'client.readonly.dir' configurable:

client.readonly.dir=clients.readonly