He ran one of the clinics where the interventions were carried out, he wasn't directly involved with the design or organisation of the trial, though I'm sure he was asked for advice...
From some old notes about Wessely's role in it PACE:
The PACE trial identifier says: "(S)taff at the Clinical Trials Unit (CTU) [...] will be primarily responsible for randomisation and database design and management [...], directed by Professor Simon Wessely, in collaboration with Professor Janet Darbyshire at the MRC CTU."
The trial protocol says: “The authors thank Professors Tom Meade, Anthony Pinching and Simon Wessely for advice about design and execution.”
The acknowledgements of the 2011 PACE paper says: “Simon Wessely commented on an early draft of the report.” And: "We thank the participants who took part in the PACE trial, staff from all the centres (including all PACE [...] specialist medical care doctors: [...] Simon Wessely."
So, as far as a formal connection to PACE goes, Wessely was one of two people "primarily responsible for randomisation and database design and management"; gave "advice about design and execution"; "commented on an early draft of the report"; and was one of the specialist medical care doctors during the trial (i.e. administered one of the trial's interventions).
Furthermore, Wessely has indisputably been one of the main architects and advocates for the hypothesis underlying the cognitive-behavioural model of ME/CFS, and the two 'treatments' based on it (CBT & GET). PACE was the big test of his baby, his big original contribution to medicine, the thing upon which he built the foundation for his career and empire.
Wessely also has close and long standing professional and ideological ties to the PACE primary investigators, and personal ones as well (as he has noted), and over the years has had an influential role in just about every governing institution, professional organisation, and advisory body of significant relevance to ME/CFS in the United Kingdom, and many international ones too.
Wessely is up to his neck in PACE, however you slice it. The distinction between formal and informal roles is a relatively minor one in this case.
References:
PACE trial identifier
http://trialsjournal.biomedcentral.com/articles/10.1186/1745-6215-10-114
PACE trial protocol 2007
http://www.biomedcentral.com/1471-2377/7/6
Main PACE paper (Lancet 2011)
http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(11)60096-2/abstract
PACE Statistical Analysis plan (2013)
http://www.trialsjournal.com/content/14/1/386