I'm interested in where the disabled fit in socialist theory and policy. Because it is probably a broad subject, I am interested specifically in what Marx thought of disabled peoples' status as workers, and how the early USSR and CCP treated them.
I can't speak to the USSR or China, but I can give you some idea of what Marx thought.
At the time Marx was writing, the disabled and disability didn't play a particularly large role in socialist thinking. The principle political object of the late 19th century socialist imaginary was the industrial able-bodied male worker, based on the belief that capitalist industrialization was subsuming the entire working-class into industrial work and that only this class had the momentum and power to overturn society.^1 This is obviously, in hindsight, a false understanding of the trajectory of industrialization, but that is the ideological context for situating Marx's ideas about disability.
To the extent that Marx talks about disability in capitalist society, it's usually in the context of industrial accidents or injury, which he attributes to the capitalist impulse towards overwork, as well as the repetitive nature of the capitalist division of labor. In particular, the chapters on the working day (chapter 10) and machinery (chapter 15) in Capital deal with this fairly extensively and scathingly:
What could be more characteristic of the capitalist mode of production than the fact that it is necessary, by Act of Parliament, to force upon the capitalist the simplest appliances for maintaining cleanliness and health?^2
According to Marx, capitalism also has the tendency to create a "relative surplus population" through the expulsion of workers from the labor process through technological unemployment (among other things); among these, he includes
the demoralized, the ragged, and those unable to work, chiefly people who succumb to their incapacity for adaptation, an incapacity which results from the division of labour; people who have lived beyond the worker's average life-span; and the victims of industry, whose number increases with the growth of dangerous machinery, of mines, chemical works, etc., the mutilated, the sickly, the widows, etc.^3
So Marx is fairly critical of capitalism's tendency to create "pauperism," as well as what he perceives as its cruelty in leaving these people to fend for themselves (since they can only survive through wage-labor, which they cannot perform for one reason or another).
So insofar as Marx was concerned about disability, it was mostly with respect to the negative effects of capitalist industry. He thought this could be ameliorated to an extent through social legislation, but only really solved through social revolution. Thus, while he was fairly sympathetic to the plight of the "victims of industry," his concern was far narrower than modern activism or writing on disability.
Regarding disability outside of capitalism, Marx critiqued the SPD (in his Critique of the Gotha Programme) for stating that workers should receive the "undiminished proceeds" of their labor by noting that a communist society would need "reserve or insurance funds to provide against accidents, dislocations caused by natural calamities, etc." as well as "funds for those unable to work, etc., in short, for what is included under so-called official poor relief today."^4
In this text we also find Marx's famous dictum about distribution and labor in communism, "From each according to his ability, to each according to his needs!"^5 What is interesting is that Marx explicitly identifies distribution based on need, rather than ability, as the hallmark of communist society; how much one receives is not based on work, since work has been qualitatively transformed and turned into "life's prime want" and wealth is supposedly non-scarce. This, however, follows from a discussion of how wealth must be distributed in "undeveloped" communism — that is, "according to labor," which he acknowledges as a fundamentally "bourgeois limitation."