Assuming that they do get summoned and do show up, how would their anonymity be preserved? Is there any precedent to bringing in an anonymous person that we can look to for an example?
Whistleblower.org has an example from a similar (though probably not as high profile) situation in the US (emphasis mine):
An IRS hearing in September 1997 illustrated the risks facing whistleblowers who try to remain anonymous. The Senate Finance Committee held a hearing about IRS enforcement of tax laws. Six witnesses described as top IRS workers were granted anonymity and testified from behind screens, their voices modified electronically, to prevent their identification.
Whistleblower Jennifer Long, a 15-year IRS tax auditor, ran into her supervisor at a Houston airport where she was catching a flight to the hearing. Her cover blown, Long testified publicly at the nationally televised hearing about “egregious tactics used by IRS revenue agents.”
Internationally, I've heard about these kinds of measures as well. For example, the International Criminal Court in the Hague has the following on protecting witnesses in the courtroom on its website:
The Chamber can order certain protective measures that apply while the witness testifies in order to protect the witness's identity and whereabouts. Such measures may consist of face/voice distortion while the witness is giving evidence and/or the use of a pseudonym. Judges can also conduct parts of hearings in private or closed sessions when necessary to protect the witness's identity or the identity of other persons at risk on account of testimony.