One thing to note is that you don't have to be an ISP to be a member of AFRINIC, since you are looking at a 10 year lease, applying for resource as end users could be a way forward, with enough justification, I am sure the block of IPs can be allocated.
http://www.afrinic.net/en/services/rs
Doubt I have any other solution I can point you to. From what I know, most local internet registries will not lease out such a block cheaply, or anyone on the internet for that matter. Last I checked some guys sell a single IPv4 address for as much as $5, you can do the math for a /22 :)
I hope the suggestion helps.
Regards,
@njukey