How can I get allowed values for an RTC enumeration list attribute using web APIs

Hello,
I have an 'Enumeration list' (multiple selection) field, but I cannot get the enumeration values for it.

When I get the resource shape, the field looks like:

<rdf:Description rdf:about="https://my.rtc:9443/ccm/oslc/context/_0u1_QV7hEeSPi_AaoZ4IKA/shapes/workitems/task/property/customMultiSelect">
        <rdf:type rdf:resource="http://open-services.net/ns/core#Property"/>
        <oslc:valueType rdf:resource="http://open-services.net/ns/core#Resource"/>
        <oslc:representation rdf:resource="http://open-services.net/ns/core#Either"/>
        <oslc:readOnly rdf:datatype="http://www.w3.org/2001/XMLSchema#boolean">false</oslc:readOnly>
        <oslc:propertyDefinition rdf:resource="http://jazz.net/xmlns/prod/jazz/rtc/ext/1.0/customMultiSelect"/>
        <oslc:occurs rdf:resource="http://open-services.net/ns/core#Zero-or-many"/>
        <oslc:name rdf:datatype="http://www.w3.org/2001/XMLSchema#string">customMultiSelect</oslc:name>
        <oslc:defaultValue>[]</oslc:defaultValue>
        <dcterms:title rdf:parseType="Literal">Custom Multi Select</dcterms:title>
</rdf:Description>

Enumeration fields (single selection) also have an <oslc:allowedValues> element, that provides an enumeration that can be looked up, but enumeration lists don't seem to have this.

If I retrieve a work item, the field doesn't show up unless it has a value set, so I can't get the possible enumerations from that either. 

How can I get the options for my attribute?

Thanks.