Something to consider on the whole tithing front: It may very well be the thing that brought them to the boards in the first place.
For many Christians, especially your southern baptist variety, talking about finances is rude. About the only time you'll ever hear it discussed as you grow up is in the context of tithing. Tithing is how responsible budgeting and personal finance management are introduced within the culture as necessary moral imperatives. In order to meet your obligation you must be deliberate in how you spend money. Younger/struggling christian people will wail at their elders, "how can I possibly give 10%? I can't afford that!"
And the elders respond, "How much do you give now? Is it 9%?, Is it 1%? No, it's 0, because you haven't made it your priority to manage yourself to meet this obligation. You spend too much, and save too little. Get your own house in order before you voice doubts upon the commands of the Lord."
There is a direct line between a conversation about how hard it would be to tithe and my own path to my current 65% savings rate.
The MMM answer to the serial tither is to demand the biblical passage that requires the 10% in cold hard cash. The MMM'er has the no-brainer answer to the tithing expense: time/services. Volunteering at the church only one day a week more than meets your tithe obligation (1/7 > 1/10). Any pastor/priest that denies this denies the word of God. Providing some necessary service for the church (bookkeeping, janitorial, landscaping, project management, cooking, decorating, music) can also go a long way towards meeting the obligation with no cash outlays.
If someone insists they have to give 10% as money, they just haven't been exposed to a serious discussion of what tithing really is, what it really means. Or they just really hate volunteering/working, and would rather spend money on that particular problem (which as OP points out, is pretty un-MMM).
If I was responding to a case study (and I rarely do) I'd just point that out. They may list it as non-negotiable, but a fundamental of MMM is that we recognize our time and freedom as MORE valuable than money. Therefore an MMM person volunteering 1/10 of their time is making a bigger sacrifice than if we tithe 1/10 of our money. Likely we're being far truer to the biblical intent as well.
Anyone wondering what to do in retirement re:tithe take note. 10% of your investment income, or 10% of your spend, isn't going to come anywhere close. The Lord demands 10% of everything. It isn't a system you can game, and only you know if you've given enough. No one can offer you absolution, and the best advice anyone can offer from a spiritual perspective is that you probably can't give too much.