Healbot: what to do when the healer can't make it

In my campaign we have seven players, only one of which is a class with any sort of healing whatsoever, a warforged cleric. No paladin, shaman, bard, etc. This makes things tough when that player can't make it. So what we do instead is have his character go into "automatic mode" and become Healbot.

Our rules for Healbot are as follows:
• Acts in initiative round 0.
• Never deals damage or attacks, and cannot be targeted/damaged by an attack. Essentially invisible to enemies.
• Moves around the battlefield to stay in range of as many allies as possible.
• Cast Healing Word (with his normal modifiers) on bloodied allies.
• Will make Heal checks to grant allies saving throws or to let them use their second wind.

Granted, this only works flavor-wise because he's a warforged, and even then only by embracing the warforged = robots theme, but...it works! Without a healer, we find that EL+0 or even EL-1 encounters are almost impossibly hard. With Healbot, they are tolerable, while still being quite challenging.