This is a very rough perl script that will go through a css file and find the possible rgba statements, it will then insert a fallback hex value before the rgba statement. As this was tailored to a particular use case, it only deals with background color

rgbafallback.pl

#!/usr/bin/perl

use strict;

sub min {
	my ($a, $b) = @_;
	if($a < $b) { return $a; }
	else { return $b; }
}

sub conv {
	my ($col, $alpha) = @_;
	return min(1, (($alpha)*$col/255 + (1-$alpha)));
}

sub convert {
	my ($r,$g,$b,$a) = @_;
	return sprintf('#%x%x%x',
				   255*conv($r, $a),
				   255*conv($g, $a),
				   255*conv($b, $a));
}

while(<>) {
		   chomp;
		   my $line = $_;
		   if($line =~ /(background(-color)?: ?rgba\(([0-9]+), ?([0-9]+), ?([0-9]+), ?([0-9.]+)\);)/) {
			   my $hex = convert($3, $4, $5, $6);
			   my $pos = index($line, $1);
			   $line= substr($line, 0, $pos)."background-color: $hex; ".substr($line, $pos);
			  }
		   if($line =~ /(border(-[^:]*)?:([^;]*)?rgba\(([0-9]+), ?([0-9]+), ?([0-9]+), ?([0-9.]+)\);)/) {
			   my $hex = convert($4, $5, $6, $7);
			   my $pos = index($line, $1);
			   $line = substr($line, 0, $pos)."border$2: $3 $hex; ".substr($line, $pos);
		   }
		   print $line . "\n";
}

usage: perl rgbafallback.pl mycss.css > bettercss.css