it's possible. the only issue is how bad do you think the
situation is. For example if this h1n1 stays as easily spread and
also as undetectable + is as deadly as sars, it might be practical
to do so.
and you don't need to qurantine those that just pass through as
they pose no danger to singaporeans. Only those that are coming
into singapore proper (none-transit) will need to be qurantined.
Add to that the decrease in number of peopl who would come in due
to these restrictions. The number will be manageable.
The main consideration is really how you weigh the risk of loss
of lives vs the economic cost of doing so. Currently the risk of
losing lives is low. You'll probably lose less than 5 lives so they
judge that the 5 lives are not worth as much as the potential
losses from the qurantining 
changi airport handles more than 35 million passengers annually
... that's almost 1000k a day .... say only half are not in transit
... 50k a day .... every day till the crisis is over ....and even
if you discount the number of people to 10% of that figure, the
really desperate ones, those who die die must come into singapore
regardless .....
where to park them ? .... you're not going to pitch tents in
football fields and run concentration camps you know ....or you're
just going to re-export them back to where they came from ? ...
but of course ... when people hear singapore's qurantining
everyone that comes in ..... regardless of flu symptoms ...
and of course ... you do realize it's really you and me and the
other tax-payers that are paying for all these eh ? ...
