Hi Scott, and welcome to OBi-land.
Yes, you are describing exactly what an OBi Analog Telephone Adapter does.
This has been discussed in length on this forum, albeit hard to find.
Short answer: you could either use your existing Google Voice number with an OBi 200 or 202, or you may be able to port your landline number into Google Voice and use that number, or use both numbers.
Google Voice doesn't accept ports in from landline carriers -- only mobile carriers. So, you'd have to first port out the AT&T number to a mobile carrier, like AT&T GoPhone, wait a week for inbound/outbound voice calls and text messages to work, then port it into Google Voice. However, even then, the number may not be portable. You can check by entering the number here:
https://www.google.com/voice/porting.
If it says "Ooops! This number appears to be from an area we don't currently support." then it won't work even if you first port it to a mobile carrier. If it says "Ooops! We currently don't support porting from your carrier. We apologize and are working on adding support for more carriers." then it should be portable after you first port to a mobile carrier.
Yes, you can use your house wiring, and yes, you should be able to get AT&T to convert your service from voice+DSL to "naked DSL". Naked DSL has no dial tone; just the DSL signal. You
MUST fully disconnect the AT&T telco wires from your 66 block, as it could damage the OBi. You'd then connect the OBi's phone jack to the 66 block, or plug it into any telephone jack in the house. The OBi should be able to support 5 phones, assuming the REN value of all the phones adds up to 5 or below.