Is normalization to a template other than MNI152 possibly within SPM12?
It is, but you need tissue probability maps (TPM) for each tissue type. If you have a template then you would need to segment it and then feed these TPM to the normalization step in SPM.
Another way would be to use nipype and set up a pipeline to do all preprocessing in SPM except the normalization step which you could do with FSL, or better yet with ANTs. You can even create your own template with ANTs based on your subject’s brains and normalize that to the MNI152 or the pediatric template. Then use the transformations to warp each brain to fit this newly created template. Nipype is a bit of a hassle to learn, though. If you have someone around who’s worked with it before that would help you a lot.
Once you have the normalized images (regardless of method, i.e. SPM or FSL or ANTs) you can load them into SPM to run the statistics on them.