Martin (1992: 109-10):
As noted in 3.1 above, English resources for signalling that the identity of a participant is recoverable include proper names, pronouns and the definite article and demonstratives. Choosing among these resources depends in part on how much experiential meaning needs to be made explicit to identify the participant in question. Proper names and pronouns are very restricted in the forms of experiential modification they allow (see Halliday (1985: 168) and so can only be selected when next to no experiential meaning is required. However, interpersonal considerations are also critical; referring to participants by name opens up an important set of interpersonal resources particularly sensitive to tenor and ideology which unfortunately cannot be pursued here.
 To be clear, a proper name does not "signal that the identity of a participant is recoverable". It actually provides the identity of the participant; no recovery is therefore required. The confusion here is between identity and identifiability.
 To be clear, these are grammatical categories, and they function as reference items in the grammatical system of reference.
 To be clear, reference is a system of the textual metafunction, and the choice is between two types of co-reference: personal (pronouns) vs demonstrative (the definite article and other specific determiners).
 To be clear, in SFL theory, modification is a function of the logical metafunction, not the experiential metafunction. Moreover, the restrictions on modification apply just as much to demonstratives as they do to proper names and pronouns. But, more to the point, any modification of reference items realises ideational meaning, not textual meaning, and so plays no part in the textual system of reference. The confusion, once again, is one of metafunction.
 The claim here is that proper names can only be selected when next to no experiential meaning is required to identify the participant, despite the fact that selecting a proper name actually provides the experiential meaning that identifies the participant.
 This confuses the interpersonal function of vocatives with the textual function of reference items.