There is nothing wrong with taping into a 3/4" mainline to start a 1" mainline for irrigation. You just want to tap into the main as close to the meter as possible. While there are other considerations (and these are the things that will limit your design to less than 10gpm), the main thing to worry about is pressure loss. The longer the water has to travel through a 3/4" mainline, the more pressure you are going to lose. For example: assuming a flow of 10 gpm, for every 100 feet of 3/4" copper (Type K) pipe, you lose over 14psi of pressure. But a 1" pipe will lose less than 4psi for the same distance.
Assuming we're talking about a mainline and meter that are burried in the yard...
How to you cut into the mainline to insert the copper tee? Since all of this is burried, you're not going to be able to push the ends of the cut the 3" apart needed to insert them into a copper tee. Even if you expose enough main line to allow some latteral seperation to like the tee on the way those instruction books show with a compression tee, the stops inside the fitting are going to prevent you from completing the connection.
Given that copper fittings have stops in them, I see only one way the geometry of this is going to work. 1. You build a special purpose fitting by connecting two 90 degree elbows (pointed in the same direction) to each end of a copper tee with short pieces of pipe (maybe even really short if you use street elbows). 2. Cut a section out of the mainline and soilder a pair of 90 degree elbows (pointed in the same direcition) such that distance between these elbows matches the distance between the elbows of your special purpose fitting. 3. Now you can soilder the elbows all together with two short pieces of pipe.
For those that might have difficulty visuallizing the finished product, follow this word picture of the flow of water. The water leaves the meter and makes a left turn followed by a right turn, through the tee for the irrigation system, then a right turn followed by a left turn, then on into the house.
Now using this many elbows seems like an ugly kluge and I hope someone has a better solition.
I have one other alternate idea, but I curious what input others have first.