SLAStation somewhat confused

Hi there,
The documentation for SLATrunk and SLAStation say that you map to a physical device through a virtual device in sla.conf and sip.conf or sla.conf and zapata.conf. But what if i dont want to dedicate a Zap channel?

My scenario is this:
I have three zap channels, Zap/1-1 - Zap/1-3. I use these channels as both outbound dial tone and inbound to IVR. I have 4 Sip phones each with their own extension and one additional shared or SLA extension. I dont need SLA until an inbound call connects or rings the proposed SLA extension. I dont want outbound calls made from the SLA extension but can deal with that as long as it uses the same Zap/G1 dial plan as I currently use. Is this possible without dedicating a Zap trunk to the SLAStation?

I noticed in the sla.pdf one can use a sip channel instead of a zap channel. Can this sip channel be a local channel or does it have to have outside dialtone? A config example would be much appreciated!
Thank you
Love Asterisk!
ps running asterisk-