Are loans with enforced repayment possible on the blockchain? Are loans with interest possible in Bitcoin, such that repayment is enforceably encoded with Bitcoin Script on the blockchain from the establishment of the lending contract?
What do you mean by "loans with interest"? If you mean guaranteeing being
payed back the principal plus some interest that is not possible since 
that depends on the external world (i.e. it is offchain).

On the other hand, if you mean just creating repayment links for the agreed amounts it is just following the instructions of BIP21.