Is it feasible to use CiviPledge as a microloan management system?
It could work, (but if there is an interest, as Laryn mentioned)

Two other tools you might want to look at:

1) civiCampaign: you could have one campaign per loan (with deadline an financial goal) and record the payments as donations associated to this campaign.

(probably easier than pledge if the frequency of the payments is not very regular)

2) civicase: if you have a lot of other activities around a microloan beside the financial one (eg if there is an approval process before the loan, if there is a set of action in case on delay on payments....)

have a look at civicase, it's a powerful tool that might take you some time to set up, but that will help you enforce consistent workflow to support your work