By 'pledge' I mean simply leaving a certain BTC value associated with a certain key-pair (for instance.) As long as that remains the case, the value is 'pledged'.
A key element of my vision here would be that BTC backers would not wish to destroy the alternate (break their pledge) because they are receiving some benefit and/or it is cumbersome or costly to do. Nor would the backers want it to be destroyed so the alternate would be designed such that users would elect to use it over other alternatives. This provides an engine driving evolution toward currency solutions which work best for everyone.
Now there are a lot of questions in the design of an alternate chain not the least of them being how and if to arrange convertibility to BTC, and there are a range of possibilities here. Again, though, I envision them having no impact on Bitcoin itself.