This has been a fascinating topic for me.
I had no idea this kind of thing was an issue, but then, I've only found in my past two years or so of blog reading that "The Stole -- inside or out?" was such a contentious subject.

Now I'm curious if there are directives about the garb of lay people taking various liturgical roles of which I have been ignorant? (And would this solve the ongoing servers-in-sneakers, lectors-in-miniskirts, extraordinary-ministers-in-shorts controvery that gets people's knickers in a twist? I'm guessing it wouldn't be any help to those embroiled in the related choir-crossing-their-legs-or-slouching-unbecomingly fracas....)
I take it the Cermonial of Bishops is the place to look for this kind of info?

(And while I'm here, a blessed Easter season, Fr. Keyes!)

these are developing practices.

for altar servers, the US Bishops have a nice general document on their website at usccb.org, go to departments, and then to Liturgy and then to Documents, finally scroll down to Guidelines for Altar Servers.

you'll find this as 6:

Acolytes, altar servers, readers, and other lay ministers may wear the alb or other suitable vesture or other appropriate or dignified clothing. (General Instruction of the Roman Missal, no.339) All servers should wear the same liturgical vesture.

the Circular Letter Concerning the Preparation and Celebration of the Easter Feasts says nothing about vesture for Passion readers or for anyone on Good Friday. however, the GIRM does say something in its section on vesture for eucharist about "other sacred actions directly connected with Mass" (337) whether Good Friday fits that description is another question!

the 3rd edition of the GIRM also says this:

336. The sacred garment common to ordained and instituted ministers of any rank is the alb, to be tied at the waist with a cincture unless it is made so as to fit even without such. Before the alb is put on, should this not completely cover the ordinary clothing at the neck, an amice should be put on. The alb may not be replaced by a surplice, not even over a cassock, on occasions when a chasuble or dalmatic is to be worn or when, according to the norms, only a stole is worn without a chasuble or dalmatic.

so, for the US, this particular law seems to supercede the general advice from the Ceremonial of Bishops.

if Brother Mike has received the ministries of acolyte and lector, as many brothers have, then 336 seems to apply to him.

if Father Jeff is ever attending a Good Friday Liturgy of the Lord's Passion not as presider or preacher, then his cassock and cross are certainly suitable. but the principle seems to be, when one serves the people of God, the vesture of the role supercedes particularities.

since we're relatively new at having other priests as concelebrants and preachers, I don't think there is anything that says specifically that a deacon or presbyter preaching a homily should be vested in alb and the appropriate stole. it certainly seems to be common practice. we have to ask if this is a fitting principle, and I agree that this may still be an open question.

about leading the Stations of the Cross:

Father Jeff, you certainly have latitude here in what you wear.

but my real question is: why would you be leading the Stations? your new parish has a wealth of deacons and lay leaders who could fittingly do this!