Just to expand on my reply in #4:
I would, personally, expect "meant seriously", and I briefly experience a "garden-path-sentence" reaction to "It was seriously meant". After a brief hesitation while I re-parse "seriously meant" as a compound adjective or adjectival phrase (like "sturdily built") following a copular verb, I decide any difference in meaning is negligible and move on. I am not sure of the chances of my parsing it that way the first time round. I have a vague recollection of reading "seriously meant" a few times before, possibly in older literature, and others may be much more familiar with the phrase; it would not surprise me to find that it is chiefly BE.
As a general rule, if the sentence intends to answer the question
"How was that meant?",
I would expect the answer
"It was meant seriously." or
"It was meant jokingly."
with a passive verb followed by an adverbial complement (similar to the active "I meant that seriously.")
In this case, I suppose, the alternative would have been more like
"It was meant as an empty political slogan."
If seriously is used as an (optional) adjunct in an adjectival phrase, it is placed in front of the adjective, as in "He was seriously ill", but that is not the case here.
So I would tend to parse "It was seriously meant." as a copular verb (was) followed by a compound adjective (seriously-meant), or, possibly, an adjectival phrase .
Perhaps the difference between "meant seriously" and "seriously meant" is comparable to the difference between "ernst gemeint" and "ernstgemeint" (before the Kultusministerkonferenz got involved).