In financial services there is a similar dynamic where service providers or consulting firms want to be able to say that a high percentage of their associates have a terminal degree or level of certification in their field, such as a CPA or MBA. Even when the employee already has all of the relevant knowledge and there will be no change in the actual service provided, it increases the firm's ability to sell its services.
If this allows the firm to increase its volume or rates, the employees who are willing to put in the extra effort to achieve certifications the company deems valuable should share in the benefits accrued company in a significant and permanent way.
If you need a high level of participation simply to compete or survive in your industry, you may need to make certification within a certain timeframe a condition of employment or advancement. If this is the case, you are really changing the nature of the job itself which is always difficult. Unless you intend to fire those who don't get certified, you have to create a compelling reason for employees to do additional work on their own time.
Some options you could try include:
1) Offer a limited number of special "Paid Time Off" days to employees to study. Many companies like mine who have a vested interest in community involvement or other activities use these types of programs. However, they typically reward people who already want to do these things, rather than inducing those who were not already interested to participate.
2) Create role-specific requirements that reflect the company's ability to earn. For example, salary bands are often capped. However, you could raise the cap for employees with certain certifications, or say that one must have certifications X, Y, and Z to become a program manager or tech lead. Unlike denying a higher annual raise based on certification, this gives the employee a choice: I can make up to $X without the cert, or up to $Y with the cert, and either will be OK with management because it reflects the difference in what the company can make from the employee. Again, the key is what the company can earn rather than what the employee can produce.
3) Hold an instructor-led exam prep class on-site. This could be a voluntary offering during "lunch" hours, which is often perceived as less intrusive, or could be full-time training for those not on projects at the time.
4) Offer additional non-financial benefits. Many corporate resources, such as training budgets or formal mentoring opportunities are often scarce by nature, forcing the company to allocate them. You can choose to put employees who pursue certs at the top of the list for things like first preference in project assignments, job rotation, mentoring opportunities, or opportunities in emerging technologies. This can be valuable to someone who wants to move into management, change roles, or learn how to work with "the hot new thing".
5) Clarify which certs you value most. If it's "everything", and the list changes every six months, people will feel they will never be able to stay current.
6) Professional status. This is much harder, but ultimately you can create a culture that communicates that you value certification. That's who we hire. That's who we promote. That's who has influence around here. However, you have to accept that a certain number of people will not fit this culture and will leave or become dissatisfied.
I would also question whether employees currently on projects need to study at the same time. Do you really want them splitting their (finite) energy between clients and studying? It may be preferable to encourage or require them to study in a more intensely focused way the next time they have "free" time.
Best of luck!