okay dumb question why not just put the line into the intake and out the return (spray bar under water)?
Some people do feed CO2 into the filter intake.
The problem with this (especially for canister filters) is that if there is sufficient CO2 being produced, then it could cause cavitation within the filter. But I doubt that DIY CO2 will generate enough CO2 to cause such problems.
For the filter return (the spray bar output), you want it to be below the surface of the water so that excessive surface agitation does not occur. However, at the same time, you want there to be some surface movement (soft rippling).