Syntax: allow newline but not blank lines in a region

I have a syntax for a markup language where certain delimited constructs
allow single newlines but not blank lines, i.e. this is OK

``` markup
{_foo
bar_}
```

but this is not OK

``` markup
{_foo

bar_}
```

is there any way to set up a region so that it behaves like this? I tried
`oneline` plus a contained `syn match` which matches a newline not preceded
or followed by a blank line but no luck!

I get the behavior I want with a `syn match` with this pattern

``` pattern
{_\%([^\n]\|\%(\_^\s*\)\@<!\n\%(\s*\_$\)\@!\)\{-}_}
```

but then highlighting obviosly doesn't kick in until I type the closing
delimiter so I would prefer a region.

Also is there a better way to match the start or end of the file than
`\_.\@<!` and `\_.\@!`?

TIA,

/bpj

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