It may depend on what your contract of employment says. Does it say that your gross pay will be topped up?
A colleague recently went on maternity leave and our company paid her while she was off. The €280 was paid directly to the employee from SW and we topped up her net by e.g. if her net salary was €500/wk we paid her a net payment of €220 each week.
If the MB payment was going to the company, we would do it exactly in the manner you have described above. If the company did not make payment in this manner, the employee would actually be better off by being on maternity leave!
AFAIK as maternity top up is not required to be paid by employers, they are entitled to top up the pay in the manner that is most beneficial to themselves. Don't forget that the company will likely have extra expense by employing someone to cover while you are off. I am currently pregnant and will be going on ML in approximately six weeks and I will be extremely happy if the company top up my net pay while I am off.