From 'The New Fowlers English Usage', page 350:
2 hardly... than. This, and the parallel constructions barely ... than and scarcely
... than, which have arisen by analogy with no sooner than, are labelled by the
OED (s.v. than conj. 3d) with the condemnatory sign ¶The examples cited are: He had scarcely won for himself the place which he deserved, than his health was found shattered-Froude, 1864; Hardly had the Council been re-opened at Trent... than Elizabeth was allying herself with the Huguenots—F. W.
Maitland, 1903. Jespersen (1909-49, vol. vii) added three more examples ('in vulgar or half-vulgar speech') from the works of minor 20c. writers. Fowler himself judged the construction to be 'surprisingly common' and cited several examples: The crocuses had hardly come into bloom in the London parks than they were swooped upon by London children; Hardly has Midsummer passed than municipal rulers all over the country have to face the task of choosing new mayors. In post-Fowlerian times both hardly ... than and (esp.) hardly ... when constructions have continued in use. Examples: (hardly ... than) Hardly were they past the carrier than two Corsairs 'scrambled' off the deck to 'intercept an enemy plane'—Daily Tel, 1944; Hardly had the chalky jet stream dissipated above the than it was ......
From the evidence before me the standard construction of the two is hardly... when; but it can only be a matter of time before the OED needs to append a usage note to its catachrestic sign s.v. than conj. 3d. Meanwhile hardly than can easily be avoided.