I love it / need it to browse my microSD card folders and play
Any supported Rockbox device has this feature. What the original firmware does or doesn't do is entirely inconsequential to Rockbox.
for instance the new Amazon.com: SanDisk Clip Sport [...snip...] doesn't have it and chipset memory is so small that we are having a hardtime porting Rockbox onto it :/
Not quite. No one is having a 'hardtime' (sic) porting Rockbox to this device, because no one is even trying to do so. It was established very, very quickly after the release of this device that Rockbox in its current form absolutely will not, under any circumstances, run on this device.
It is possible that one day someone might make a third party firmware for this device, and it is highly probable that if someone did that they would use Rockbox as a starting point (it would be prudent to re-use the bootloader, playback engine, and codecs), but the fact of the matter is that the end result would be entirely unrecognizable as Rockbox.
The Rockbox theme engine would be entirely out of the question, and the plugin and viewers subsystem is almost certainly out of the question as well.
[Saint]