I don't find this very flexible. If you look at what happened with LACNIC, countries from ARIN were transferred to LACNIC. I expect that when AFRINIC is activated, countries from both RIPE and ARIN will be transferred to AFRINIC.
In general this did not happen with LACNIC. ARIN had been allocating IPv4 space out of 200 /8. The remainder of the /8 was transferred to LACNIC at activation. ARIN tried to get a separate /23 of v6 space for LACNIC allocations but was refused by ICANN/IANA, hence allocation were made from an ARIN /23. LACNIC has since recieved their own /23 and are working with those ISPs to renumber. In regards to AfriNIC, ARIN has tried to allocate out of a block for just AfriNIC for v4. ARIN requested a separate /23 for v6 but was refused by ICANN/IANA. Ray