I think you will find that quite a lot of travellers do go south to Padang or fly into Padang from Singapore.
There are lots of lovely beaches south of Bukittinggi and places to stay.
There is one special place I discovered in 2000 when I was on a bus from Bengkulu to Padang. My indonesian friend and I stopped off in Painan and found a great little beach house 2kms out of the village owned by a character called Eddy Usmardi.
It has become a favourite haunt of mine and I usually visit every year.
Painan is 2 hrs by bus from Padang. Best to get off in the main village and ask someone and for a short bus ride you will be dropped outside Eddy's Homestay.
Here is the link to a series of posts I wrote last year re Painan forum.indahnesia.com/topic/170(...)nan_west_sumatra.php
About 4-5 posts down is a link to the photos I took of the area plus all the other photos I took during my 60 days in Indonesia- Jan/Feb 2005.
As to the National Park I cannot help you as I have never been there.
Suggest you post on Lonely Planet Thorn Tree.
There is a young Indonesian guy who posts there under the name of donabollo?? who actually has family who live in Lampung or thereabouts.
He may be able to help you and Laszlo from Thorn Tree should have some knowledge of the area as he has travelled extensively throughout Indonesia visiting all the true 'off the beaten track places- National Parks etc.
Plus another experienced traveller to Sumatra who posts regualrly on Thorn Tree is RedApe. he should be able to contribute to your queries.