There are some pear grafts onto Mountain Ash here in central Alberta that have lasted for years and fruit regularly. Unfortunately it doesn’t work this way for me. My conclusion is you have to be lucky to get a mountain ash/pear variety combo that is compatible for the longer term. They exist for sure, but for me its way too “hit and miss” to continue grafting pears to Mountain Ash in my area. Hopefully you will have long term compatibility, which is certainly possible. Good luck on your experiments!