I will look into writing a function that can create an excerpt on the fly, I don't want to revive that legacy field in the blog.
Please be aware that if you are working with the source code I recommend you do not make changes in the mojoPortal projects but put all customizations in external projects. If you propose a change that makes it back into the project thats fine, but if you want to make changes that will not be integrated back in, you should do it in your own projects, otherwise you are forking the code and it will be difficult to get updates without losing your custom changes. So for example if you propose a change to the blog that I don't agree with but you still want to do it, then you should clone the blog into your own custom projects, changing the namespace, changing the table prefix from mp_ to something else. I had to do this once before myself when I had a customer that wanted a blog with some custom functionality.
I think you mentioned in another post about using the blog like a news list. Have you tried putting a blog on a different page and then add an RSS feed to the home page and consume the feed from your blog into the RSS feature. I think the RSS feature could use some improvements but is a better tool for this job. I think it may have a setting already for just showing links or maybe we can add the calculated excerpt there.