I’m not sure this is a thing – having volunteers doing building LCAs.
Any company that does LCA work for projects is liable for that work. No one will immediately die if you get it wrong (it’s not structural engineering) but there is significant reputational risk at stake – either for greenwashing a carbon intensive project, or deviating from standards and compromising the viability of a project.
Also, there is a growing industry of professionals taking on this work, in return for fair wages. If you are a building LCA expert, why would you try to volunteer, effectively undercutting those professionals? I would suggest to put your hat in the ring for an LCA consultancy role 
Get paid a fair and honest wage for that time.