rem-fallback.less

@main-font-size: 16px;

.x-rem (@property, @value) {

  // This is a workaround, inspired by https://github.com/borodean/less-properties
  @px-fallback: @value * @main-font-size;
  -: ~`(function () { return ';@{property}: @{px-fallback}'; }())`;
  -: ~`(function () { return ';@{property}: @{value}rem'; }())`;
}

.some-class {
  .x-rem(font-size, 1.4);
}

rem-fallback.scss

$main-font-size: 16px;

@mixin x-rem ($property, $value) {
  #{$property}: $value * $main-font-size;
  #{$property}: #{$value}rem;
}

.some-class {
    @include x-rem(font-size, 1.4);
}