Meta-analysis using GWAS datasets of different diseases

Question

It is well known that in Meta-analysis of Genome-wide association studies (GWAS) we combine the results of independent studies to get a pooled effect estimates (Odd ratios or betas) in order to improve the association signals of rare variants to uncover new candidate genes. In the literature is very common to find meta-analysis of several traits or diseases, like meta-analysis of Alzheimer's disease, Schizophrenia, bipolar disorder, diabetes, etc. But I did not find any meta-analysis using GWAS of different disease, combining all in just one, for example, a meta-analysis using datasets of Cohrn's disease, Rehumatoid Artritis and Tipe I diabetes (all autoimmune diseases) in order to find association signals of common variants across these diseases. I know that combining same phenotypes is a rule of thumb because it helps to avoid the precense of heterogeneity between studies, but if we have GWAS of different diseases with similar population structure, genotyping plataforms and these diseases are known to share thier etiology in a certain way, it is valid to perform such meta-analysis?, can I use the inverce variance model (using Odd ratios and Standar errors)? or maybe there is other way to pool these results? I hope I made clear my questions, thank you in advance!