An ugly hack to use rems with pixel fallbacks for any CSS property.

remedy.less

//
// Usage:
//
//     @import 'remedy.less';
//     @remedy-base-size: 16px;
//     .something {
//         .remedy('font-size'; 12px);
//         .remedy('margin'; 1.2rem 3rem);
//     }
// 
// Two versions of each property will be created: one using rems and one using
// pixels.

.remedy(@property, @value) {
	// LESS doesn't have a good way to do this, so we need to do something kinda
	// like SQL injection: create a value with a terminator (';') followed by
	// new statements.
	-remedy-dummy: ~`(function() {
		return '1; '
			// The value for our dummy property
			+ @{property} + ': '

			// The real property. This will be the px version.
			+ "@{value}"
				.replace(/[\[\],]/g, '')

				// Replace any rem lengths with their pixel equivalents.
				.replace(/\d+(\.\d+)?rem/g, function(v) {
					return ''
						+ (parseFloat(v) * parseFloat('@{remedy-base-size}'))
						+ 'px';
				})
			+ '; '

			// The real property again. This time for the rem version.
			+ @{property} + ': '
			+ "@{value}"
				.replace(/[\[\],]/g, '')

				// Replace any pixel lengths with their rem equivalents.
				.replace(/\d+(\.\d+)?px/g, function(v) {
					return ''
						+ (parseFloat(v) / parseFloat('@{remedy-base-size}'))
						+ 'rem';
				});
	}())`;
}

2 cents, I think remedy.less should include @remedy-base-size: 16px; in the less. That way using it doesn't require the var be set, but can be redeclared as needed.