The real value added by a cleric is in the healing, and in the buffs.
What you want to do is tie together your other party members, add the option for some healing (with a cost) and not have to put up with a complete NPC character.
So you make a custom item.
The item has the following properties ...
a) there are a series of bracelets, each one bound to the next, which provide a group-controlled magic item, of sorts. Each character wears a bracelet, but the properties are such that it activates another bracelet.
b) The bracelets provide the healing Powers of a cleric, of whatever appropriate level you wish.
c) At a minimum, the bracelets can activate two instances of the "Healing Word" power per encounter on another bracelet (not the activator's). The calculation includes the bonus to the "Healing" keyword that a cleric normally gets, as well as the regular bonus healing from the Healing Word power.
d) When the party would have access to 2nd level spells, add one instance of "Cure Light Wounds" to the bracelet's power pool.
e) when a cleric character would normally gain access to a "Cure BLANK wounds" spell, or another major healing milestone spell, simply add it to the bracelet's power structure.
Now, the penalties ...
It is up to you exactly how you wish to penalize the party for using the bracelet and not have access to a *real* cleric.
Some ideas ....
a) each use of the "Healing Word" powers costs the activator one hitpoint. Each use of a "Cure BLANK Wounds" spell costs the activator 5 hitpoints.
or
b) activating the braclet has a 10 percent chance of draining a healing surge on the activator as well as the character being healed. (5 percent for the "Healing Word" powers)
or
c) the bracelets require a ritual, cast every day, to keep them powered up. This ritual has expensive components, and activating higher level powers costs even more (calculated out to be approximately a 1/2 share when compared to the rest of the party).
Basically, this whole setup gives the party access to the healing aspects, a little pause to think everytime they use it, and yet allowed everybody to function without an actual extra character (netting them extra XP and loot).