Martin (1992: 131):
Circumstances of Extent are realised through nominal groups realising measurement: yards, laps, rounds, years etc. (Halliday 1985: 137-138). It is unlikely that they would be presumed by a phoric nominal group:
[3:68] She swam 40 laps
Actor Process Circumstance: extent
To be clear, Martin has defined 'participant' in terms of nominal groups functioning as the Agent or Medium of a clause. This definition already excludes all circumstances.
But even in Martin's terms (ideational denotation), such circumstances do "introduce a participant" whose identity can be subsequently presumed, as shown by:
She swam 40 laps. They were her fastest yet.