I checked my SK on live the other day and the breastplate was inventory clickable. I figure Donal's is inventory clickable so makes sense that BE & others might be too.
I searched around a bit and never found anything about this being 'must equip' to cast at any time aside from eqmac wiki. Though eqmac wiki shows donal's breastplate as must equip.
http://www.eqmacwiki.com/w/Donal%27s...te_of_Mourning
Perhaps someone can help confirm if certain breastplates are must equip and others inventory or whether they were all must equip until a certain point.