In the context of a Brexit delay, an article in the Guardian mentions that
The prospect of a lengthy delay has been made more palatable after the UK’s advocate general in the European court of justice, Eleanor Sharpston, described claims that the European elections in May would be an “insuperable obstacle” to a lengthy extension as “oversimplified and fallacious”. [...]
Sharpston said the UK could extend the mandates of its MEPs “who have already been democratically elected and who have been sitting in the current European parliament” or send “nominated MPs from the UK, rather than directly elected MEPs, to participate in the European parliament during that period”.
Is there precedent in the EU for MPs from the national assembly of a member EU country to be sent as MEPs without an[other] election specific for the latter purpose? Well, the answer to this question is somewhat trivial yes: that was actually the case when the UK joined the EU (well EC)... and the situation lasted surprisingly long—six years, from 1973 till 1979. But the catch is that the 1979 election was the first direct election of MEPs across the EU. So it makes more sense to ask: were national MPs (from a EU country) sent as MEPs without a dedicated EU election after 1979?