If you fork the code you will have no way to upgrade without losing your customizations to mojoportal code. There is nothing I can do to change that or solve it. If you do that then you and your customer will have to live with the consequences. I recommend explain the consequences and convince the customer about the down side of doing that. I can promise that sooner or later you or your customer will regret not being able to upgrade, there will be a new feature you want or a security fix or something.
Your options are as follows:
Wait until I get around to it which may be a while.
You can try to maintain your fork by creating patches of your changes and then try to re-apply them to the latest mojoportal code from time to time to get the latest mojoportal code, but over the long haul that will be a pain. whether that is viable depends on whether the customer will be a long term paying customer or whether you just do the work and move on. If you provide a fork that your customer cannot upgrade and then move on it will turn out badly one day when they get someone else to upgrade who doesn't know about your changes.
You can possibly convince me to prioritize the change by convincing your customer to sponsor the effort financially to make the changes. But my guess is they will not want to pay what it would take to convince me to drop my other priorities and do this right now.
You can possibly propose and develop a solution that meets my approval and contribute it to the project. Note however that the solution cannot be integrated unless it supports all the databases we support. you could possibly implement it for the db you use and convince your customer to sponsor my time to implement the other data layers so it can be integrated. To contribute, you would have to sign and return a contributor agreement. If your employer owns copyright to your work then a valid representative of the employer must also sign it.
So, the bottom line is people cannot get everything they want for free or cheap and sometimes have to accept what is reasonable and possible within their budget.