Hello, I know nothing about citrus… other than I’d love to be able to grow them (outside) here in Oregon! …But here are my random thoughts about ‘under-planting’ fruit trees (and I suspect there will be better/others to follow).
Most of the time you want tree roots going as deep as possible for water; with ‘flowers,’ requiring near constant summer watering, the tree roots will ‘turn up’ toward that moisture. If and when you remove the flowers and stop the frequent watering the tree may suffer as it’s become shallow-rooted.
Also, without ‘deep rooting’ high winds may blow them over easier.
Otherwise, a shade loving flower like Begonias might do well… That’s a good question, one I don’t know that we discussed here… Anyone else 
There are some highly experienced citrus and tropical fruit growers in the Tampa Bay Rare Fruit Council, as well as in many of the organizations they link to from here: http://www.rarefruit.org/linkmain.htm
That said, I will pass on one bit of experience I've gained this weekend: do not plant anything thorny under your fruit trees. I knew I was pruning a neglected apple tree -- didn't realize until I backed into one of the dozen rose bushes under it just WHY it had been neglected! In Florida, I'd especially avoid planting prickly pear as well.
Best of luck -- and let us know what you find out!
Idyllwild
simplepress
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Marsha H
Viron
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