Just a final update for anyone who might have stumbled across this with a similar problem.
Last week I re-piped this heating system using the suggestions from here and some slight modifications along the way. While the BTU savings might be minimal (I believe wood burning is more efficient than before), the result is that I now at least have options for heating that I didn't have before.
Here's a rundown of what I did:
First, I teed into the baseboard return line above the shutoff valve (#1) that is located just above the circulator. I ran this to a ball valve (#2), and then teed it into the wood boiler return line. There was an existing valve to isolate this line from the oil system (#3)
I then ran a line from the wood boiler hot line to a ball valve (#4), and then teed it into the baseboard return valve just below the shutoff valve (#1) but above the circulator. There was an existing valve (#5) that isolates the wood boiler's hot supply from the oil side.
There is also an existing valve on the return just before it goes into the oil furnace. (#6)
So here's how it works...
Primary Wood + Oil Backup
: Valves #1 + #3 + #5 closed, and valves #2 + #4 + #6 open. Water returning from the baseboards is routed into the return (bottom) on the wood boiler, then out from the boiler to the circulator back into the oil furnace and back into the system. If the water in the oil furnace goes below 160 degrees the oil burner kicks on as necessary. This is how I run the system when I am only burning small fires a couple times a day.
: Same as above but with oil burner switched off (circulators still on). This is how I will run the system once I start keeping the fire going all the time. This prevents the oil furnace from running at all -- which at $3.00+ per gallon is very desirable! Of course, I've got to remember to switch on the oil when I'm leaving the house for a while.
Primary Oil + Wood Backup
: Valves #2 + #4 closed, and #1 + #3 + #5 + #6 open. This runs the wood boiler in "passive" mode where it only circulates by gravity feed/hydronic flow...how it was installed originally. I honestly don't know why I would ever run it like this.
: Valves #2, #4, #3 + #5 closed and #1 and #6. The wood boiler is completely isolated from the system.
: With the power out, a 7th valve (electronic normally open) opens and allows gravity circulation directly to/from the baseboard loop. The hot water goes out through a dedicated pipe from this valve and returns through whichever return valve happens to be open. This takes place automatically, thankfully.
The weather here has turned cold (20's at night, 40s daytime) so I have been running it in the primary wood + oil backup configuration with great results!