By the sounds of your question/case it sounds as though your patient has DI. But you have also stated 'enamel hypoplasia', therefore is the situation DI or AI? If it is AI, I would recommend looking into the literature regarding 5% Sodium Hypochlorite pre-treatment of enamel prior to bonding in order to remove/denature proteins from the crystaline enamel structure. If your case however is DI, the effectiveness of dentin bonding - is unknown - and will be an issue for you, in areas of dentine. But in areas of enamel, again still in the DI scenario, bonding anything (brackets, composites, veneers) will be done so with the risk of enamel shearing away from the dentine. Further, your concerns regarding obliterated or filled in pulp spaces and reduced areas of enamel from attrition are pointing towards crown coverage and then providing orthodontic attachments to those. I hope I have answered some aspects of your question. I agree strongly with Agnieszka above for an interdisciplinary approach. Please post how it all goes.

In case of traditional bonding use self-adhesive cement like Fuji cement to enhance

bonding strength to to dentin

Is an DI or AI?

For AI usually we use 5% Naocl for treatment before bonding.

For DI interdisciplinary approach is needed as my colleagues reported with crowns, and if porcelain then use hydrochloric acid.

If you are sure about the cooperation and the motivation of the patient about the oral hygiene procedures, it is better to use a self-etch ahesive bonding agent such as Kuraray Clearfill SE Bond which were reported to be the gold standard for bonding to dentine for years now.  If you need to restore the teeth with crown restorations, it is not logic to deliver definitive ceramic restorations before orthodontic treatment. In this case it would be better to use some long term provisional crowns which were cemented to the teeth permanently. For this purpose, the best choice among the provisional crown materials in the market would be a provisional CAD-CAM block, such as Ivoclar Vivadent Telio CAD blocks. They are though enough for long term use and could be polished very well. And you can easily bond the brackets on these kind of PMMA or composite materials.Or you can use some other more conventional materials as well. Ask to your dental technician about the possibilities please. Only at the end of orthodontic treatment, it would be possible to fabricate the ceramic definitives...

Correction to my answer. Hydrofluoric acid if porcelain crowns 

How about placing SS bands cemented with glass ionomer cement. Pre banding , use iodine solution with NaF varnish 3 Xs and then q3 months during orthodontic phase.

The iodine/NaF varnish will knock down the lactobacilli to zero and maybe preserve the exposed dentin.

I would clean dentin surface with pomice and chlorexidine. Then I will apply a 2 step self etch system.

the first step is a self-etching primer and should be applied with active brushing for a long time. Then it should be dried with air (etch and dry procedure). Then the second step (bonding) should be applied and cured after solvent evaporation.

If and where enamel is still present it should selectively pre-etched with 37% H3PO4 acid for 30 seconds.

Now self etch composites are in market. Where slight conditioning of dentine followed by composite application. Composite is by kerr company.

SS bands in anterior region affects the aesthetics during treatment.

In such cases we can use crown on anterior teeth and SS bands on posterior teeth if direct bonding is not possible.