There is no standard way of doing this.
But I agree with your idea. I have done the same thing, sending the planned orders to PIR inactive version in ECC that is connected to a scenario for LTP.
But why not just transfer the DP forecast from APO to the "inactive" PIR version in ECC?
Then run LTP?
I have also created a program to transfer the SNP planned orders to ECC inactive version.
The reason why people do this: LTP allows you to run financial reports that are already being utilized w/in ECC. Also not every product/component may be planned in APO.
SAP should develop this capability but it is easy to do this using ABAP or LCOUT.