We end up with customers asking for more IPs too. We just add additional subnets to the interface, perhaps they started with a /30 but now need three more IPs, we just add an additional /29 to the interface leaving both blocks.
It is not often that anything needs to be explained to the customer other than the correct subnet mask and gateway for the IPs. This makes our configs look like this for each customer vlan:
ip address 22.214.171.124 255.255.255.252
ip address 126.96.36.199 255.255.255.224 secondary
That being said, I know at least one of our transit customers does hosting exactly how you are describing. Coincidentally, this customer is also one of the customers that asked if we could "give them a class C block."
Using this strategy has never been a problem with ARIN for us, in fact I have applied for and received more space at intervals between 6 and 14 months for the last four years without any issue at all.